Basic biostatistics

1. Rate
Sensitivity
Predictive value
Sujit Kumar Kar

2. Epidemiological research
 Need
 Definition of what is to be measured
 Clear and precise
 Valid
 Acceptable and applicable for large population
 Establishment of criteria and standards by
which it can be measured

3. Measurements in epidemiology
 Mortality
 Morbidity
 Disability
 Presence or absence or distribution of
characteristics
 Demographic variables

4. Basic needs of measurement
 Validity
 Reliability
 Accuracy
 Sensitivity
 Specificity

5. Tools of measurement
 Disease magnitude is often expressed in
terms of
 Rates
 Ratios
 Proportions

6. Rate
 A rate measures the occurrence of some
particular event in a population during a given
time period
 It is a statement of the risk of developing a
condition
 It indicates the change in some event in a
given population over a period of time

7. Rate
 Expressed as
 Rate = number of event /population in that time
period× 1000
 Eg – death rate, birth rate
 Elements in a “rate”
 Numerator
 Denominator
 Time specification
 Multiplier

8. Rate
 Types
 Crude rates (Unstandardized rates) – birth rate
, death rate
 Specific rates – (Due to specific cause or
occurring in specific groups or in specific time
periods) – MMR, IMR
 Standardized rates (obtained by direct or
indirect method of standardization)- age and
sex standardized rated

9. Ratios
 Expresses a relation in size between two
random quantities
 The numerator is not a component of the
denominator
 X: Y = X/Y
 Eg- doctor-population ratio, male – female
ratio

10. Proportion
 A proportion is a ratio which indicates the
relation in magnitude of a part of the whole
 The numerator is always included in the
denominator
 Always expressed in terms of percentage
 Eg- IQ

11. Incidence
 The number of new cases occurring in a
defined population in a specified period of
time
 Incidence = Number of new cases/ population
at risk (during the defined period) × 1000

12. Prevalence
 The term disease prevalence refers to all
current cases (old and new) existing at a
given point of time or over a period of time in
a given population
 Although referred as rate, it is a ratio
 Two types
 Point prevalence
 Period prevalence

13. Relation between incidence and
prevalence
 Prevalence = Incidence X Duration
 Longer is the duration of the disease, greater
is the prevalence
 Disease of acute onset or with rapid recovery
or death and short duration is associated with
low prevalence

14. Evaluation of screening test
 Screening test result by diagnosis
 Sensitivity = proportion of positives that are correctly
identified by a test = probability of a positive test, given
the patient is ill.
 Specificity = proportion of negatives that are
correctly identified by a test = probability of a negative
test, given that patient is well.
 Positive predictive value is the proportion of patients
with positive test results who are correctly diagnosed.
 Negative predictive value is the proportion of
patients with negative test results who are correctly
diagnosed.

15. Evaluation of screening test

16. Screening
test results
Diagnosis Total
Diseased Not diseased
Positive True positive
(a)
False positive
(b)
a+b
Negative False negative
(c)
True negative
(d)
c+d
Total a+ c b+d a+b+c+d
Sensitivity = a/(a+c)× 100
Specificity = d/(b+d)× 100
Positive predictive value = a/(a+b)× 100
Negative predictive value = d/(c+d)× 100

17. Sensitivity
 The term was introduced by – Yerushalmy in
1940s as a statistical index of diagnostic
accuracy
 Definition – the ability of a test to identify
correctly all those who have a disease (true
positive)

18. Specificity
 Definition – the ability of the test to identify
correctly those who do not have the disease
(true negative)

19.  An ideal screening test should be 100 percent
sensitive and 100 percent specific, this
seldom occurs

20. Predictive value
 Performance of a screening test is measured
by its “predictive value”
 It reflects the diagnostic power of the test
 Predictive accuracy depends upon –
 Sensitivity
 Specificity
 Disease prevalence

21. Predictive value
 Predictive value of a positive test – indicates
the probability that a patient with a positive
test result has, in fact disease in question
 More prevalent a disease – more accurate will
be the predictive value of a positive screening
test
 Predictive value of a positive result falls as
prevalence declines

22. Thank You