This document discusses simple statistical tests for analyzing data, including confidence intervals, p-values, and chi-square tests. It describes the goals of understanding confidence intervals and p-values and learning basic statistical tests. It defines types of variables and measures of association. It provides examples of calculating odds ratios, confidence intervals, and chi-square tests. It explains how to select the appropriate statistical test based on sample size and expected cell counts and provides examples from public health studies.

1. Data Analysis:
Simple Statistical Tests

2. Goals
 Understand confidence intervals and p-
values
 Learn to use basic statistical tests
including chi square and ANOVA

3. Types of Variables
 Types of variables indicate which estimates you can
calculate and which statistical tests you should use
 Continuous variables:
 Always numeric
 Generally calculate measures such as the mean, median
and standard deviation
 Categorical variables:
 Information that can be sorted into categories
 Field investigation – often interested in dichotomous or
binary (2-level) categorical variables
 Cannot calculate mean or median but can calculate risk

4. Measures of Association
 Strength of the association between two variables,
such as an exposure and a disease
 Two measure of association used most often are the
relative risk, or risk ratio (RR), and the odds ratio
(OR)
 The decision to calculate an RR or an OR depends
on the study design
 Interpretation of RR and OR:
 RR or OR = 1: exposure has no association with disease
 RR or OR > 1: exposure may be positively associated with
disease
 RR or OR < 1: exposure may be negatively associated with
disease

5. Risk Ratio or Odds Ratio?
 Risk ratio
 Used when comparing outcomes of those who were
exposed to something to those who were not exposed
 Calculated in cohort studies
 Cannot be calculated in case-control studies because the
entire population at risk is not included in the study
 Odds ratio
 Used in case-control studies
 Odds of exposure among cases divided by odds of exposure
among controls
 Provides a rough estimate of the risk ratio

6. Analysis Tool: 2x2 Table
 Commonly used with dichotomous
variables to compare groups of people
 Table puts one dichotomous variable
across the rows and another
dichotomous variable along the
columns
 Useful in determining the association
between a dichotomous exposure and a
dichotomous outcome

7. Calculating an Odds Ratio
 Table displays data from a case control study conducted in
Pennsylvania in 2003 (2)
 Can calculate the odds ratio:
 *OR = ad = (218)(85) = 19.6
bc (45)(21)
Outcome
Exposure
Hepatitis A
No Hepatitis
A
Total
Ate salsa 218 45 263
Did not eat
salsa
21 85 106
Total 239 130 369
Table 1. Sample 2x2 table for Hepatitis A at Restaurant A

8. Confidence Intervals
 Point estimate – a calculated estimate (like
risk or odds) or measure of association (risk
ratio or odds ratio)
 The confidence interval (CI) of a point
estimate describes the precision of the
estimate
 The CI represents a range of values on either side
of the estimate
 The narrower the CI, the more precise the point
estimate (3)

9. Confidence Intervals - Example
 Example—large bag of 500 red, green and
blue marbles:
 You want to know the percentage of green
marbles but don’t want to count every marble
 Shake up the bag and select 50 marbles to give
an estimate of the percentage of green marbles
 Sample of 50 marbles:

15 green marbles, 10 red marbles, 25 blue marbles

10. Confidence Intervals - Example
 Marble example continued:
 Based on sample we conclude 30% (15 out of 50)
marbles are green
 30% = point estimate
 How confident are we in this estimate?
 Actual percentage of green marbles could be
higher or lower, ie. sample of 50 may not reflect
distribution in entire bag of marbles
 Can calculate a confidence interval to
determine the degree of uncertainty

11. Calculating Confidence Intervals
 How do you calculate a confidence interval?
 Can do so by hand or use a statistical
program
 Epi Info, SAS, STATA, SPSS and Episheet are
common statistical programs
 Default is usually 95% confidence interval but
this can be adjusted to 90%, 99% or any
other level

12. Confidence Intervals
 Most commonly used confidence interval is the 95%
interval
 95% CI indicates that our estimated range has a 95%
chance of containing the true population value
 Assume that the 95% CI for our bag of marbles
example is 17-43%
 We estimated that 30% of the marbles are green:
 CI tells us that the true percentage of green marbles is most
likely between 17 and 43%
 There is a 5% chance that this range (17-43%) does not
contain the true percentage of green marbles

13. Confidence Intervals
 If we want less chance of error we could
calculate a 99% confidence interval
 A 99% CI will have only a 1% chance of error but
will have a wider range
 99% CI for green marbles is 13-47%
 If a higher chance of error is acceptable we
could calculate a 90% confidence interval
 90% CI for green marbles is 19-41%

14. Confidence Intervals
 Very narrow confidence intervals indicate a very
precise estimate
 Can get a more precise estimate by taking a larger
sample
 100 marble sample with 30 green marbles

Point estimate stays the same (30%)

95% confidence interval is 21-39% (rather than 17-43% for
original sample)
 200 marble sample with 60 green marbles

Point estimate is 30%

95% confidence interval is 24-36%
 CI becomes narrower as the sample size increases

15. Confidence Intervals
 Returning to example of Hepatitis A in a
Pennsylvania restaurant:
 Odds ratio = 19.6
 95% confidence interval of 11.0-34.9 (95% chance that the
range 11.0-34.9 contained the true OR)
 Lower bound of CI in this example is 11.0 (e.g., >1)

Odds ratio of 1 means there is no difference between the two
groups, OR > 1 indicates a greater risk among the exposed
 Conclusion: people who ate salsa were truly more likely to
become ill than those who did not eat salsa

16. Confidence Intervals
 Must include CIs with your point estimates to give a
sense of the precision of your estimates
 Examples:
 Outbreak of gastrointestinal illness at 2 primary schools in
Italy (4)

Children who ate corn/tuna salad had 6.19 times the risk of
becoming ill as children who did not eat salad

95% confidence interval: 4.81 – 7.98
 Pertussis outbreak in Oregon (5)

Case-patients had 6.4 times the odds of living with a 6-10 year-
old child than controls

95% confidence interval: 1.8 – 23.4
 Conclusion: true association between exposure and disease
in both examples

17. Analysis of Categorical Data
 Measure of association (risk ratio or
odds ratio)
 Confidence interval
 Chi-square test
 A formal statistical test to determine
whether results are statistically significant

18. Chi-Square Statistics
 A common analysis is whether Disease X
occurs as much among people in Group A as
it does among people in Group B
 People are often sorted into groups based on their
exposure to some disease risk factor
 We then perform a test of the association between
exposure and disease in the two groups

19. Chi-Square Test: Example
 Hypothetical outbreak of Salmonella on
a cruise ship
 Retrospective cohort study conducted
 All 300 people on cruise ship interviewed,
60 had symptoms consistent with
Salmonella
 Questionnaires indicate many of the case-
patients ate tomatoes from the salad bar

20. Chi-Square Test: Example (cont.)
 To see if there is a statistical difference in the amount
of illness between those who ate tomatoes (41/130)
and those who did not (19/170) we could conduct a
chi-square test
Salmonella?
Yes No Total
Tomatoes 41 89 130
No Tomatoes 19 151 170
Total 60 240 300
Table 2a. Cohort study: Exposure to tomatoes and Salmonella infection

21. Chi-Square Test: Example (cont.)
 To conduct a chi-square the following
conditions must be met:
 There must be at least a total of 30 observations
(people) in the table
 Each cell must contain a count of 5 or more
 To conduct a chi-square test we compare the
observed data (from study results) with the
data we would expect to see

22. Chi-Square Test: Example (cont.)
Salmonella?
Yes No Total
Tomatoes 130
No Tomatoes 170
Total 60 240 300
 Gives an overall distribution of people who ate tomatoes and
became sick
 Based on these distributions we can fill in the empty cells
with the expected values
Table 2b. Row and column totals for tomatoes and Salmonella infection

23. Chi-Square Test: Example (cont.)
 Expected Value = Row Total x Column Total
Grand Total
 For the first cell, people who ate tomatoes and
became ill:
 Expected value = 130 x 60 = 26
300
 Same formula can be used to calculate the expected
values for each of the cells

24. Chi-Square Test: Example (cont.)
Salmonella?
Yes No Total
Tomatoes
130 x 60 = 26
300
130 x 240 = 104
300
130
No Tomatoes
170 x 60 = 34
300
170 x 240 = 136
300
170
Total 60 240 300
 To calculate the chi-square statistic you use the observed values
from Table 2a and the expected values from Table 2c
 Formula is [(Observed – Expected)2
/Expected] for each cell of
the table
Table 2c. Expected values for exposure to tomatoes

25. Chi-Square Test: Example (cont.)
Salmonella?
Yes No Total
Tomatoes
(41-26)2
= 8.7
26
(89-104)2
= 2.2
104
130
No Tomatoes
(19-34)2
= 6.6
34
(151-136)2
= 1.7
136
170
Total 60 240 300
 The chi-square (χ2) for this example is 19.2
 8.7 + 2.2 + 6.6 + 1.7 = 19.2
Table 2d. Expected values for exposure to tomatoes

26. Chi-Square Test
 What does the chi-square tell you?
 In general, the higher the chi-square value,
the greater the likelihood there is a
statistically significant difference between the
two groups you are comparing
 To know for sure, you need to look up the p-
value in a chi-square table
 We will discuss p-values after a discussion of
different types of chi-square tests

27. Types of Chi-Square Tests
 Many computer programs give different
types of chi-square tests
 Each test is best suited to certain
situations
 Most commonly calculated chi-square
test is Pearson’s chi-square
 Use Pearson’s chi-square for a fairly large
sample (>100)

28. Types of Statistical Tests
Parade of
Statistics Guys
The right test...
To use when….
Pearson chi-square (uncorrected) Sample size >100
Expected cell counts > 10
Yates chi-square (corrected) Sample size >30
Expected cell counts ≥ 5
Mantel-Haenszel chi-square Sample size > 30
Variables are ordinal
Fisher’s exact test Sample size < 30 and/or
Expected cell counts < 5

29. Using Statistical Tests:
Examples from Actual Studies
 In each study, investigators chose the type of test
that best applied to the situation (Note: while the chi-
square value is used to determine the corresponding
p-value, often only the p-value is reported.)
 Pearson (Uncorrected) Chi-Square : A North Carolina study
investigated 955 individuals because they were identified as
partners of someone who tested positive for HIV. The study
found that the proportion of partners who got tested for HIV
differed significantly by race/ethnicity (p-value <0.001). The
study also found that HIV-positive rates did not differ by
race/ethnicity among the 610 who were tested (p = 0.4). (6)

30. Using Statistical Tests:
Examples from Actual Studies
 Additional examples:
 Yates (Corrected) Chi-Square: In an outbreak of Salmonella
gastroenteritis associated with eating at a restaurant, 14 of
15 ill patrons studied had eaten the Caesar salad, while 0 of
11 well patrons had eaten the salad (p-value <0.01). The
dressing on the salad was made from raw eggs that were
probably contaminated with Salmonella. (7)
 Fisher’s Exact Test: A study of Group A Streptococcus
(GAS) among children attending daycare found that 7 of 11
children who spent 30 or more hours per week in daycare
had laboratory-confirmed GAS, while 0 of 4 children
spending less than 30 hours per week in daycare had GAS
(p-value <0.01). (8)

31. P-Values
 Using our hypothetical cruise ship Salmonella
outbreak:
 32% of people who ate tomatoes got Salmonella
as compared with 11% of people who did not eat
tomatoes
 How do we know whether the difference
between 32% and 11% is a “real” difference?
 In other words, how do we know that our chi-
square value (calculated as 19.2) indicates a
statistically significant difference?
 The p-value is our indicator

32. P-Values
 Many statistical tests give both a
numeric result (e.g. a chi-square value)
and a p-value
 The p-value ranges between 0 and 1
 What does the p-value tell you?
 The p-value is the probability of getting the
result you got, assuming that the two
groups you are comparing are actually the
same

33. P-Values
 Start by assuming there is no difference in outcomes
between the groups
 Look at the test statistic and p-value to see if they
indicate otherwise
 A low p-value means that (assuming the groups are the
same) the probability of observing these results by chance is
very small

Difference between the two groups is statistically significant
 A high p-value means that the two groups were not that
different
 A p-value of 1 means that there was no difference between
the two groups

34. P-Values
 Generally, if the p-value is less than
0.05, the difference observed is
considered statistically significant, ie.
the difference did not happen by
chance
 You may use a number of statistical
tests to obtain the p-value
 Test used depends on type of data you
have

35. Chi-Squares and P-Values
 If the chi-square statistic is small, the observed and
expected data were not very different and the p-value
will be large
 If the chi-square statistic is large, this generally
means the p-value is small, and the difference could
be statistically significant
 Example: Outbreak of E. coli O157:H7 associated
with swimming in a lake (1)
 Case-patients much more likely than controls to have taken
lake water in their mouth (p-value =0.002) and swallowed
lake water (p-value =0.002)
 Because p-values were each less than 0.05, both exposures
were considered statistically significant risk factors

36. Note: Assumptions
 Statistical tests such as the chi-square assume that
the observations are independent
 Independence: value of one observation does not influence
value of another
 If this assumption is not true, you may not use the
chi-square test
 Do not use chi-square tests with:
 Repeat observations of the same group of people (e.g. pre-
and post-tests)
 Matched pair designs in which cases and controls are
matched on variables such as sex and age

37. Analysis of Continuous Data
 Data do not always fit into discrete categories
 Continuous numeric data may be of interest
in a field investigation such as:
 Clinical symptoms between groups of patients
 Average age of patients compared to average age
of non-patients
 Respiratory rate of those exposed to a chemical
vs. respiratory rate of those who were not exposed

38. ANOVA
 May compare continuous data through
the Analysis Of Variance (ANOVA) test
 Most statistical software programs will
calculate ANOVA
 Output varies slightly in different programs
 For example, using Epi Info software:

Generates 3 pieces of information: ANOVA
results, Bartlett’s test and Kruskal-Wallis test

39. ANOVA
 When comparing continuous variables between
groups of study subjects:
 Use a t-test for comparing 2 groups
 Use an f-test for comparing 3 or more groups
 Both tests result in a p-value
 ANOVA uses either the t-test or the f-test
 Example: testing age differences between 2 groups
 If groups have similar average ages and a similar
distribution of age values, t-statistic will be small and the p-
value will not be significant
 If average ages of 2 groups are different, t-statistic will be
larger and p-value will be smaller (p-value <0.05 indicates
two groups have significantly different ages)

40. ANOVA and Bartlett’s Test
 Critical assumption with t-tests and f-tests: groups
have similar variances (e.g., “spread” of age values)
 As part of the ANOVA analysis, software conducts a
separate test to compare variances: Bartlett’s test for
equality of variance
 Bartlett’s test:
 Produces a p-value
 If Bartlett’s p-value >0.05, (not significant) OK to use ANOVA
results
 Bartlett’s p-value <0.05, variances in the groups are NOT
the same and you cannot use the ANOVA results

41. Kruskal-Wallis Test
 Kruskal-Wallis test: generated by Epi Info
software
 Used only if Bartlett’s test reveals variances
dissimilar enough so that you can’t use ANOVA
 Does not make assumptions about variance,
examines the distribution of values within each
group
 Generates a p-value

If p-value >0.05 there is not a significant difference
between groups

If p-value < 0.05 there is a significant difference between
groups

42. Analysis of Continuous Data
Figure 1. Decision tree for analysis of continuous data.
Bartlett’s test for equality of variance
p-value >0.05?
YES NO
Use ANOVA test Use Kruskal-Wallis
test test
p<0.05 p>0.05 p<0.05 p>0.05
Difference between groups is
statistically significant
Difference between groups is
statistically significant
Difference between groups is
NOT statistically significant
Difference between groups is
NOT statistically significant

43. Conclusion
 In field epidemiology a few calculations and
tests make up the core of analytic methods
 Learning these methods will provide a good
set of field epidemiology skills.
 Confidence intervals, p-values, chi-square tests,
ANOVA and their interpretations
 Further data analysis may require methods to
control for confounding including matching
and logistic regression

44. References
1. Bruce MG, Curtis MB, Payne MM, et al. Lake-associated
outbreak of Escherichia coli O157:H7 in Clark County,
Washington, August 1999. Arch Pediatr Adolesc Med.
2003;157:1016-1021.
2. Wheeler C, Vogt TM, Armstrong GL, et al. An outbreak of
hepatitis A associated with green onions. N Engl J Med.
2005;353:890-897.
3. Gregg MB. Field Epidemiology. 2nd ed. New York, NY: Oxford
University Press; 2002.
4. Aureli P, Fiorucci GC, Caroli D, et al. An outbreak of febrile
gastroenteritis associated with corn contaminated by Listeria
monocytogenes. N Engl J Med. 2000;342:1236-1241.

45. References
5. Schafer S, Gillette H, Hedberg K, Cieslak P. A community-wide
pertussis outbreak: an argument for universal booster vaccination.
Arch Intern Med. 2006;166:1317-1321.
6. Centers for Disease Control and Prevention. Partner counseling and
referral services to identify persons with undiagnosed HIV --- North
Carolina, 2001. MMWR Morb Mort Wkly Rep.2003;52:1181-1184.
7. Centers for Disease Control and Prevention. Outbreak of Salmonella
Enteritidis infection associated with consumption of raw shell eggs,
1991. MMWR Morb Mort Wkly Rep. 1992;41:369-372.
8. Centers for Disease Control and Prevention. Outbreak of invasive
group A streptococcus associated with varicella in a childcare center --
Boston, Massachusetts, 1997. MMWR Morb Mort Wkly Rep.
1997;46:944-948.