SlideShare a Scribd company logo
1 of 21
Download to read offline
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Non-parametric tests
An introduction
Dr. S. A. Rizwan, M.D.,
Public Health Specialist,
Saudi Board of Preventive Medicine,
Riyadh, Kingdom of Saudi Arabia.
1Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Outline
• Some common terms
• Difference between parametric and
nonparametric tests
• When to use NPT
• Advantages & disadvantages
• Commonly used NPT
• Take home messages
2Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Introduction
• Parametric methods
– They are based in means, standard deviations or
parameters of distributions
• The Normal distribution is not always appropriate
– To study variables with a few observations
– Non-symmetrical distributions
3Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Parametric & nonparametric concept
• Most of the statistical methods referred to as parametric
require the use of interval- or ratio-scaled data.
• Nonparametric methods are often the only way to analyze
nominal or ordinal data and draw statistical conclusions.
• Nonparametric methods require no assumptions about the
population probability distributions.
• Nonparametric methods are often called distribution-free
methods.
4Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Parametric test procedures
• Involve population parameters (mean)
• Have stringent assumptions (normality)
• Examples: Z test, t test
5Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Nonparametric test procedures
• Do not involve population parameters
• Data measured on any scale (ratio or interval, ordinal or
nominal)
• Example: Wilcoxon rank sum test
6Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Assumptions of t test
• The sampling distribution is normally distributed
– For N <30 if the sample data is normally distributed then
the sampling distribution will also be normal
• The data should come from an interval or ratio scale
• In practice an ordinal scale with 5 or more levels is ok
7Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Examples of nonnormal distributions
8Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Assumptions of t test (contd.)
• There should not be extreme scores or outliers, because these
have a disproportionate influence on the mean and the
variance
• For the independent samples t test the variance in the two
samples should be approximately equal
– This assumption is more important if sample size < 30 and / or sample
sizes are unequal
– As a rule of thumb, if the variance of one group is 3 or more times
greater than the variance of the other group, then use non-parametric
9Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
What are nonparametric tests?
• ‘Non-parametric’ tests were developed for these situations
where fewer assumptions have to be made.
• Sometimes called distribution-free tests.
• NP tests can be applied to Normal data but parametric tests
have greater power if assumptions are met.
10Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
How to check for normality of data?
1. Thumb rules
a) Mean & Range & SD
b) Skewness and kurtosis
c) Compare mean, median &
mode
d) Trimmed mean
e) Outliers
2. Graphs
a) Histogram with theoretical
normal curve
b) QQ plot
c) Box plot and outlier
detection
d) Stem and leaf plot
3. Formal statistical tests
a) W/S test
b) Jarque-Bera test
c) Shapiro-Wilks test
d) Kolmogorov-Smirnov test
e) D’Agostino test
f) Grubbs and Dixon test (for
outliers)
4. Comparing non-parametric
and parametric test results
11Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Distribution-free tests
• Distribution-free tests are statistical tests that do not rely on
any underlying assumptions about the probability distribution
of the sampled population.
• The branch of inferential statistics devoted to distribution-free
tests is called nonparametrics.
• Nonparametric statistics (or tests) based on the ranks of
measurements are called rank statistics (or rank tests).
12Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Ranks
• Practical differences between parametric and NP are that NP
methods use the ranks of values rather than the actual values
• E.g.
– 1,2,3,4,5,7,13,22,38,45 - actual
– 1,2,3,4,5,6,7,8,9,10 - rank
13Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Rules for NPT
• In general, for a statistical method to be classified as
nonparametric, it must satisfy at least one of the
following conditions.
– The method can be used with nominal data.
– The method can be used with ordinal data.
– The method can be used with interval or ratio data when
no assumption can be made about the population
probability distribution.
14Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Parametric / Nonparametric tests
Parametric Tests Non-parametric Tests
Single sample t-test Wilcoxon-signed rank test
Paired sample t-test Paired Wilcoxon-signed rank
2 independent samples t-test Mann-Whitney test (Wilcoxon Rank Sum)
One-way Analysis of Variance Kruskal-Wallis
Pearson’s correlation Spearman Rank
Repeated Measures Friedman
… many more … many more
15Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Types of NPT
16Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Advantages of nonparametric tests
• Used with all scales
• Easier to compute, developed originally before wide
computer use
• Make fewer assumptions
• Need not involve population parameters
• Results may be as exact as parametric procedures
17Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Disadvantages of nonparametric tests
• May waste information
– Parametric model more efficient if data permit
• Difficult to compute by hand for large samples
• Tables not widely available
• Used only to test hypotheses, not for estimation purposes
18Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Limitations of nonparametric methods
• Converting ratio level data to ordinal ranked data entails a loss
of information
• This reduces the sensitivity of the non-parametric test
compared to the parametric alternative in most circumstances
– sensitivity is the power to reject the null hypothesis, given that it is
false in the population
– lower sensitivity gives a higher type 2 error rate
• Many parametric tests have no non-parametric equivalent
– e.g. Two way ANOVA, where two IV’s and their interaction are
considered simultaneously
19Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
Take home messages
• Fewer assumptions than parametric tests
• So useful when these assumptions not met
• Often used when sample size is small and difficult to tell if
normally distributed
• Ragbag of tests developed over time with no consistent
framework
20Nov 2019
Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA
Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course
THANK YOU
Kindly email your queries to sarizwan1986@outlook.com
21Nov 2019

More Related Content

What's hot

What's hot (20)

Parametric Statistical tests
Parametric Statistical testsParametric Statistical tests
Parametric Statistical tests
 
Statistical tests of significance and Student`s T-Test
Statistical tests of significance and Student`s T-TestStatistical tests of significance and Student`s T-Test
Statistical tests of significance and Student`s T-Test
 
Sample Size Determination
Sample Size DeterminationSample Size Determination
Sample Size Determination
 
Calculating p value
Calculating p valueCalculating p value
Calculating p value
 
COR-RELATIONAL DESIGN
COR-RELATIONAL DESIGNCOR-RELATIONAL DESIGN
COR-RELATIONAL DESIGN
 
DIstinguish between Parametric vs nonparametric test
 DIstinguish between Parametric vs nonparametric test DIstinguish between Parametric vs nonparametric test
DIstinguish between Parametric vs nonparametric test
 
tests of significance
tests of significancetests of significance
tests of significance
 
Steps in Developing A Valid and Reliable Scale.pdf
 Steps in Developing A Valid and Reliable Scale.pdf Steps in Developing A Valid and Reliable Scale.pdf
Steps in Developing A Valid and Reliable Scale.pdf
 
Parametric and nonparametric test
Parametric and nonparametric testParametric and nonparametric test
Parametric and nonparametric test
 
Parametric versus non parametric test
Parametric versus non parametric testParametric versus non parametric test
Parametric versus non parametric test
 
What is a paired samples t test
What is a paired samples t testWhat is a paired samples t test
What is a paired samples t test
 
Ch4 Confidence Interval
Ch4 Confidence IntervalCh4 Confidence Interval
Ch4 Confidence Interval
 
Normal Distribution, Skewness and kurtosis
Normal Distribution, Skewness and kurtosisNormal Distribution, Skewness and kurtosis
Normal Distribution, Skewness and kurtosis
 
Sample size estimation
Sample size estimationSample size estimation
Sample size estimation
 
Non parametric tests by meenu
Non parametric tests by meenuNon parametric tests by meenu
Non parametric tests by meenu
 
5 essential steps for sample size determination in clinical trials slideshare
5 essential steps for sample size determination in clinical trials   slideshare5 essential steps for sample size determination in clinical trials   slideshare
5 essential steps for sample size determination in clinical trials slideshare
 
P value, Power, Type 1 and 2 errors
P value, Power, Type 1 and 2 errorsP value, Power, Type 1 and 2 errors
P value, Power, Type 1 and 2 errors
 
t distribution, paired and unpaired t-test
t distribution, paired and unpaired t-testt distribution, paired and unpaired t-test
t distribution, paired and unpaired t-test
 
Tests of significance
Tests of significanceTests of significance
Tests of significance
 
sampling error.pptx
sampling error.pptxsampling error.pptx
sampling error.pptx
 

Similar to A introduction to non-parametric tests

4-191125033339.pptx kolmogorov smirnov test
4-191125033339.pptx kolmogorov smirnov test4-191125033339.pptx kolmogorov smirnov test
4-191125033339.pptx kolmogorov smirnov test
JayKeluskar1
 
Articulo 1
Articulo 1Articulo 1
Articulo 1
rcm1990
 
Study Design to Support Reimbursement
Study Design to Support ReimbursementStudy Design to Support Reimbursement
Study Design to Support Reimbursement
Lyssa Friedman
 
Developing the Reimbursement Story 2016-03-10
Developing the Reimbursement Story 2016-03-10Developing the Reimbursement Story 2016-03-10
Developing the Reimbursement Story 2016-03-10
Lyssa Friedman
 
articles in healthcare
articles in healthcarearticles in healthcare
articles in healthcare
prof beso
 

Similar to A introduction to non-parametric tests (20)

Checking for normality (Normal distribution)
Checking for normality (Normal distribution)Checking for normality (Normal distribution)
Checking for normality (Normal distribution)
 
4-191125033339.pptx kolmogorov smirnov test
4-191125033339.pptx kolmogorov smirnov test4-191125033339.pptx kolmogorov smirnov test
4-191125033339.pptx kolmogorov smirnov test
 
Analysis of small datasets
Analysis of small datasetsAnalysis of small datasets
Analysis of small datasets
 
Kolmogorov Smirnov good-of-fit test
Kolmogorov Smirnov good-of-fit testKolmogorov Smirnov good-of-fit test
Kolmogorov Smirnov good-of-fit test
 
Moderator analysis in meta-analysis
Moderator analysis in meta-analysisModerator analysis in meta-analysis
Moderator analysis in meta-analysis
 
Introduction & rationale for meta-analysis
Introduction & rationale for meta-analysisIntroduction & rationale for meta-analysis
Introduction & rationale for meta-analysis
 
Data extraction/coding and database structure in meta-analysis
Data extraction/coding and database structure in meta-analysisData extraction/coding and database structure in meta-analysis
Data extraction/coding and database structure in meta-analysis
 
Types of correlation coefficients
Types of correlation coefficientsTypes of correlation coefficients
Types of correlation coefficients
 
Methodology Checklist : Studies of Diagnostic Accuracy
Methodology Checklist : Studies of Diagnostic AccuracyMethodology Checklist : Studies of Diagnostic Accuracy
Methodology Checklist : Studies of Diagnostic Accuracy
 
Fixed-effect and random-effects models in meta-analysis
Fixed-effect and random-effects models in meta-analysisFixed-effect and random-effects models in meta-analysis
Fixed-effect and random-effects models in meta-analysis
 
Statistical tests for data involving quantitative data
Statistical tests for data involving quantitative dataStatistical tests for data involving quantitative data
Statistical tests for data involving quantitative data
 
DISEASE PREDICTION SYSTEM USING DATA MINING
DISEASE PREDICTION SYSTEM USING  DATA MININGDISEASE PREDICTION SYSTEM USING  DATA MINING
DISEASE PREDICTION SYSTEM USING DATA MINING
 
Articulo 1
Articulo 1Articulo 1
Articulo 1
 
Study Design to Support Reimbursement
Study Design to Support ReimbursementStudy Design to Support Reimbursement
Study Design to Support Reimbursement
 
Developing the Reimbursement Story 2016-03-10
Developing the Reimbursement Story 2016-03-10Developing the Reimbursement Story 2016-03-10
Developing the Reimbursement Story 2016-03-10
 
Overview of the systematic review process
Overview of the systematic review processOverview of the systematic review process
Overview of the systematic review process
 
Biases in meta-analysis
Biases in meta-analysisBiases in meta-analysis
Biases in meta-analysis
 
How evidence affects clinical practice in egypt
How evidence affects clinical practice in egyptHow evidence affects clinical practice in egypt
How evidence affects clinical practice in egypt
 
Choosing a statistical test
Choosing a statistical testChoosing a statistical test
Choosing a statistical test
 
articles in healthcare
articles in healthcarearticles in healthcare
articles in healthcare
 

More from Rizwan S A

More from Rizwan S A (19)

Introduction to scoping reviews
Introduction to scoping reviewsIntroduction to scoping reviews
Introduction to scoping reviews
 
Sources of demographic data 2019
Sources of demographic data 2019Sources of demographic data 2019
Sources of demographic data 2019
 
Effect sizes in meta-analysis
Effect sizes in meta-analysisEffect sizes in meta-analysis
Effect sizes in meta-analysis
 
Presenting the results of meta-analysis
Presenting the results of meta-analysisPresenting the results of meta-analysis
Presenting the results of meta-analysis
 
Heterogeneity in meta-analysis
Heterogeneity in meta-analysisHeterogeneity in meta-analysis
Heterogeneity in meta-analysis
 
Inverse variance method of meta-analysis and Cochran's Q
Inverse variance method of meta-analysis and Cochran's QInverse variance method of meta-analysis and Cochran's Q
Inverse variance method of meta-analysis and Cochran's Q
 
Kruskal Wallis test, Friedman test, Spearman Correlation
Kruskal Wallis test, Friedman test, Spearman CorrelationKruskal Wallis test, Friedman test, Spearman Correlation
Kruskal Wallis test, Friedman test, Spearman Correlation
 
Mantel Haenszel methods in epidemiology (Stratification)
Mantel Haenszel methods in epidemiology (Stratification) Mantel Haenszel methods in epidemiology (Stratification)
Mantel Haenszel methods in epidemiology (Stratification)
 
Use of checklists in critical appraisal of health literature
Use of checklists in critical appraisal of health literatureUse of checklists in critical appraisal of health literature
Use of checklists in critical appraisal of health literature
 
Critical Appraisal of health literature
Critical Appraisal of health literatureCritical Appraisal of health literature
Critical Appraisal of health literature
 
Critical Appraisal of health literature
Critical Appraisal of health literatureCritical Appraisal of health literature
Critical Appraisal of health literature
 
Critical Appraisal of health literature - an overview
Critical Appraisal of health literature - an overviewCritical Appraisal of health literature - an overview
Critical Appraisal of health literature - an overview
 
Evidence based medicine or health practice
Evidence based medicine or health practiceEvidence based medicine or health practice
Evidence based medicine or health practice
 
Epidemiology: Standardisation of rates
Epidemiology: Standardisation of ratesEpidemiology: Standardisation of rates
Epidemiology: Standardisation of rates
 
Statistical tests for categorical data
Statistical tests for categorical dataStatistical tests for categorical data
Statistical tests for categorical data
 
Confidence intervals: Types and calculations
Confidence intervals: Types and calculationsConfidence intervals: Types and calculations
Confidence intervals: Types and calculations
 
Confidence Intervals: Basic concepts and overview
Confidence Intervals: Basic concepts and overviewConfidence Intervals: Basic concepts and overview
Confidence Intervals: Basic concepts and overview
 
Chi square test and its types
Chi square test and its typesChi square test and its types
Chi square test and its types
 
Student's t test and variations
Student's t test and variationsStudent's t test and variations
Student's t test and variations
 

Recently uploaded

Failure to thrive in neonates and infants + pediatric case.pptx
Failure to thrive in neonates and infants  + pediatric case.pptxFailure to thrive in neonates and infants  + pediatric case.pptx
Failure to thrive in neonates and infants + pediatric case.pptx
claviclebrown44
 
Histology of Epithelium - Dr Muhammad Ali Rabbani - Medicose Academics
Histology of Epithelium - Dr Muhammad Ali Rabbani - Medicose AcademicsHistology of Epithelium - Dr Muhammad Ali Rabbani - Medicose Academics
Histology of Epithelium - Dr Muhammad Ali Rabbani - Medicose Academics
MedicoseAcademics
 
Connective Tissue II - Dr Muhammad Ali Rabbani - Medicose Academics
Connective Tissue II - Dr Muhammad Ali Rabbani - Medicose AcademicsConnective Tissue II - Dr Muhammad Ali Rabbani - Medicose Academics
Connective Tissue II - Dr Muhammad Ali Rabbani - Medicose Academics
MedicoseAcademics
 

Recently uploaded (20)

ESC HF 2024 Spotlights Day-2.pptx heart failure
ESC HF 2024 Spotlights Day-2.pptx heart failureESC HF 2024 Spotlights Day-2.pptx heart failure
ESC HF 2024 Spotlights Day-2.pptx heart failure
 
Hi Fi * Surat ℂall Girls Surat Dumas Road 8527049040 WhatsApp AnyTime Best Su...
Hi Fi * Surat ℂall Girls Surat Dumas Road 8527049040 WhatsApp AnyTime Best Su...Hi Fi * Surat ℂall Girls Surat Dumas Road 8527049040 WhatsApp AnyTime Best Su...
Hi Fi * Surat ℂall Girls Surat Dumas Road 8527049040 WhatsApp AnyTime Best Su...
 
DR. Neha Mehta Best Psychologist.in India
DR. Neha Mehta Best Psychologist.in IndiaDR. Neha Mehta Best Psychologist.in India
DR. Neha Mehta Best Psychologist.in India
 
Sell 5cladba adbb JWH-018 5FADB in stock
Sell 5cladba adbb JWH-018 5FADB in stockSell 5cladba adbb JWH-018 5FADB in stock
Sell 5cladba adbb JWH-018 5FADB in stock
 
Vesu + ℂall Girls Serviℂe Surat (Adult Only) 8849756361 Esℂort Serviℂe 24x7 C...
Vesu + ℂall Girls Serviℂe Surat (Adult Only) 8849756361 Esℂort Serviℂe 24x7 C...Vesu + ℂall Girls Serviℂe Surat (Adult Only) 8849756361 Esℂort Serviℂe 24x7 C...
Vesu + ℂall Girls Serviℂe Surat (Adult Only) 8849756361 Esℂort Serviℂe 24x7 C...
 
Gallbladder Double-Diverticular: A Case Report المرارة مزدوجة التج: تقرير حالة
Gallbladder Double-Diverticular: A Case Report  المرارة مزدوجة التج: تقرير حالةGallbladder Double-Diverticular: A Case Report  المرارة مزدوجة التج: تقرير حالة
Gallbladder Double-Diverticular: A Case Report المرارة مزدوجة التج: تقرير حالة
 
How to buy 5cladba precursor raw 5cl-adb-a raw material
How to buy 5cladba precursor raw 5cl-adb-a raw materialHow to buy 5cladba precursor raw 5cl-adb-a raw material
How to buy 5cladba precursor raw 5cl-adb-a raw material
 
duus neurology.pdf anatomy. phisiology///
duus neurology.pdf anatomy. phisiology///duus neurology.pdf anatomy. phisiology///
duus neurology.pdf anatomy. phisiology///
 
Unlocking Holistic Wellness: Addressing Depression, Mental Well-Being, and St...
Unlocking Holistic Wellness: Addressing Depression, Mental Well-Being, and St...Unlocking Holistic Wellness: Addressing Depression, Mental Well-Being, and St...
Unlocking Holistic Wellness: Addressing Depression, Mental Well-Being, and St...
 
Treatment Choices for Slip Disc at Gokuldas Hospital
Treatment Choices for Slip Disc at Gokuldas HospitalTreatment Choices for Slip Disc at Gokuldas Hospital
Treatment Choices for Slip Disc at Gokuldas Hospital
 
Failure to thrive in neonates and infants + pediatric case.pptx
Failure to thrive in neonates and infants  + pediatric case.pptxFailure to thrive in neonates and infants  + pediatric case.pptx
Failure to thrive in neonates and infants + pediatric case.pptx
 
Histology of Epithelium - Dr Muhammad Ali Rabbani - Medicose Academics
Histology of Epithelium - Dr Muhammad Ali Rabbani - Medicose AcademicsHistology of Epithelium - Dr Muhammad Ali Rabbani - Medicose Academics
Histology of Epithelium - Dr Muhammad Ali Rabbani - Medicose Academics
 
Renal Replacement Therapy in Acute Kidney Injury -time modality -Dr Ayman Se...
Renal Replacement Therapy in Acute Kidney Injury -time  modality -Dr Ayman Se...Renal Replacement Therapy in Acute Kidney Injury -time  modality -Dr Ayman Se...
Renal Replacement Therapy in Acute Kidney Injury -time modality -Dr Ayman Se...
 
Report Back from SGO: What’s the Latest in Ovarian Cancer?
Report Back from SGO: What’s the Latest in Ovarian Cancer?Report Back from SGO: What’s the Latest in Ovarian Cancer?
Report Back from SGO: What’s the Latest in Ovarian Cancer?
 
CONGENITAL HYPERTROPHIC PYLORIC STENOSIS by Dr M.KARTHIK EMMANUEL
CONGENITAL HYPERTROPHIC PYLORIC STENOSIS  by Dr M.KARTHIK EMMANUELCONGENITAL HYPERTROPHIC PYLORIC STENOSIS  by Dr M.KARTHIK EMMANUEL
CONGENITAL HYPERTROPHIC PYLORIC STENOSIS by Dr M.KARTHIK EMMANUEL
 
Young & Hot Surat ℂall Girls Dindoli 8527049040 WhatsApp AnyTime Best Surat ℂ...
Young & Hot Surat ℂall Girls Dindoli 8527049040 WhatsApp AnyTime Best Surat ℂ...Young & Hot Surat ℂall Girls Dindoli 8527049040 WhatsApp AnyTime Best Surat ℂ...
Young & Hot Surat ℂall Girls Dindoli 8527049040 WhatsApp AnyTime Best Surat ℂ...
 
NDCT Rules, 2019: An Overview | New Drugs and Clinical Trial Rules 2019
NDCT Rules, 2019: An Overview | New Drugs and Clinical Trial Rules 2019NDCT Rules, 2019: An Overview | New Drugs and Clinical Trial Rules 2019
NDCT Rules, 2019: An Overview | New Drugs and Clinical Trial Rules 2019
 
Signs It’s Time for Physiotherapy Sessions Prioritizing Wellness
Signs It’s Time for Physiotherapy Sessions Prioritizing WellnessSigns It’s Time for Physiotherapy Sessions Prioritizing Wellness
Signs It’s Time for Physiotherapy Sessions Prioritizing Wellness
 
Young & Hot ℂall Girls Patna 8250077686 WhatsApp Number Best Rates of Patna ℂ...
Young & Hot ℂall Girls Patna 8250077686 WhatsApp Number Best Rates of Patna ℂ...Young & Hot ℂall Girls Patna 8250077686 WhatsApp Number Best Rates of Patna ℂ...
Young & Hot ℂall Girls Patna 8250077686 WhatsApp Number Best Rates of Patna ℂ...
 
Connective Tissue II - Dr Muhammad Ali Rabbani - Medicose Academics
Connective Tissue II - Dr Muhammad Ali Rabbani - Medicose AcademicsConnective Tissue II - Dr Muhammad Ali Rabbani - Medicose Academics
Connective Tissue II - Dr Muhammad Ali Rabbani - Medicose Academics
 

A introduction to non-parametric tests

  • 1. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Non-parametric tests An introduction Dr. S. A. Rizwan, M.D., Public Health Specialist, Saudi Board of Preventive Medicine, Riyadh, Kingdom of Saudi Arabia. 1Nov 2019
  • 2. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Outline • Some common terms • Difference between parametric and nonparametric tests • When to use NPT • Advantages & disadvantages • Commonly used NPT • Take home messages 2Nov 2019
  • 3. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Introduction • Parametric methods – They are based in means, standard deviations or parameters of distributions • The Normal distribution is not always appropriate – To study variables with a few observations – Non-symmetrical distributions 3Nov 2019
  • 4. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Parametric & nonparametric concept • Most of the statistical methods referred to as parametric require the use of interval- or ratio-scaled data. • Nonparametric methods are often the only way to analyze nominal or ordinal data and draw statistical conclusions. • Nonparametric methods require no assumptions about the population probability distributions. • Nonparametric methods are often called distribution-free methods. 4Nov 2019
  • 5. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Parametric test procedures • Involve population parameters (mean) • Have stringent assumptions (normality) • Examples: Z test, t test 5Nov 2019
  • 6. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Nonparametric test procedures • Do not involve population parameters • Data measured on any scale (ratio or interval, ordinal or nominal) • Example: Wilcoxon rank sum test 6Nov 2019
  • 7. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Assumptions of t test • The sampling distribution is normally distributed – For N <30 if the sample data is normally distributed then the sampling distribution will also be normal • The data should come from an interval or ratio scale • In practice an ordinal scale with 5 or more levels is ok 7Nov 2019
  • 8. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Examples of nonnormal distributions 8Nov 2019
  • 9. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Assumptions of t test (contd.) • There should not be extreme scores or outliers, because these have a disproportionate influence on the mean and the variance • For the independent samples t test the variance in the two samples should be approximately equal – This assumption is more important if sample size < 30 and / or sample sizes are unequal – As a rule of thumb, if the variance of one group is 3 or more times greater than the variance of the other group, then use non-parametric 9Nov 2019
  • 10. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course What are nonparametric tests? • ‘Non-parametric’ tests were developed for these situations where fewer assumptions have to be made. • Sometimes called distribution-free tests. • NP tests can be applied to Normal data but parametric tests have greater power if assumptions are met. 10Nov 2019
  • 11. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course How to check for normality of data? 1. Thumb rules a) Mean & Range & SD b) Skewness and kurtosis c) Compare mean, median & mode d) Trimmed mean e) Outliers 2. Graphs a) Histogram with theoretical normal curve b) QQ plot c) Box plot and outlier detection d) Stem and leaf plot 3. Formal statistical tests a) W/S test b) Jarque-Bera test c) Shapiro-Wilks test d) Kolmogorov-Smirnov test e) D’Agostino test f) Grubbs and Dixon test (for outliers) 4. Comparing non-parametric and parametric test results 11Nov 2019
  • 12. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Distribution-free tests • Distribution-free tests are statistical tests that do not rely on any underlying assumptions about the probability distribution of the sampled population. • The branch of inferential statistics devoted to distribution-free tests is called nonparametrics. • Nonparametric statistics (or tests) based on the ranks of measurements are called rank statistics (or rank tests). 12Nov 2019
  • 13. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Ranks • Practical differences between parametric and NP are that NP methods use the ranks of values rather than the actual values • E.g. – 1,2,3,4,5,7,13,22,38,45 - actual – 1,2,3,4,5,6,7,8,9,10 - rank 13Nov 2019
  • 14. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Rules for NPT • In general, for a statistical method to be classified as nonparametric, it must satisfy at least one of the following conditions. – The method can be used with nominal data. – The method can be used with ordinal data. – The method can be used with interval or ratio data when no assumption can be made about the population probability distribution. 14Nov 2019
  • 15. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Parametric / Nonparametric tests Parametric Tests Non-parametric Tests Single sample t-test Wilcoxon-signed rank test Paired sample t-test Paired Wilcoxon-signed rank 2 independent samples t-test Mann-Whitney test (Wilcoxon Rank Sum) One-way Analysis of Variance Kruskal-Wallis Pearson’s correlation Spearman Rank Repeated Measures Friedman … many more … many more 15Nov 2019
  • 16. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Types of NPT 16Nov 2019
  • 17. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Advantages of nonparametric tests • Used with all scales • Easier to compute, developed originally before wide computer use • Make fewer assumptions • Need not involve population parameters • Results may be as exact as parametric procedures 17Nov 2019
  • 18. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Disadvantages of nonparametric tests • May waste information – Parametric model more efficient if data permit • Difficult to compute by hand for large samples • Tables not widely available • Used only to test hypotheses, not for estimation purposes 18Nov 2019
  • 19. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Limitations of nonparametric methods • Converting ratio level data to ordinal ranked data entails a loss of information • This reduces the sensitivity of the non-parametric test compared to the parametric alternative in most circumstances – sensitivity is the power to reject the null hypothesis, given that it is false in the population – lower sensitivity gives a higher type 2 error rate • Many parametric tests have no non-parametric equivalent – e.g. Two way ANOVA, where two IV’s and their interaction are considered simultaneously 19Nov 2019
  • 20. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course Take home messages • Fewer assumptions than parametric tests • So useful when these assumptions not met • Often used when sample size is small and difficult to tell if normally distributed • Ragbag of tests developed over time with no consistent framework 20Nov 2019
  • 21. Saudi Board of Preventive Medicine, Riyadh Ministry of Health, KSA Dr. S. A. Rizwan, M.D.Demystifying statistics series: Meta-analysis course THANK YOU Kindly email your queries to sarizwan1986@outlook.com 21Nov 2019