SAMPLE SIZE

STATISTICAL SYMBOLSN = Total populationn = Sample SizeL, d, e = Margin of error or absolute

precision or Relative precisionP = prevalence or Anticipated

population proportionP Value = Probability or Chance

Sample SizeThe sample size calculation depends on:

Type of study

Magnitude of the outcome of interest derived from

previous studies.

Type of statistical analysis required (comparing

means or proportions)

Level of significance / power.

 

NULL HYPOTHESISAny hypothesis which has to be tested for possible rejection under assumption it is true is called null hypothesis.All analytical study designs require hypothesis.Do state your hypothesisSleeping pills Thalidomide leads to congenital malformation.

3. Choosing an appropriate statisticst test, z test for continuous data, chi square for proportions etc.

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Test statistics is computed from the sample data and is used to determine whetherthenull hypothesis should be rejected orretained.

Test statistics generates p value

STEPS IN HYPOTHESIS TESTING

Simply put, p-value is the chance that the relationship you are observing is observed by pure chance.  So if you obtain a p-value of 0.01, it means that there is a 1% chance that the relationship between the independent variable(s) and the dependent variable that your model established doesn't actually exist.

HYPOTHESIS IS TESTED TO RULE OUT ELEMENT OF CHANCE. AND TO FIND TRUE DIFFERENCE BETWEEN TWO GROUPS

IF P = 0.05 THEN CONFIDENCE LEVEL WOULD BE 95%

IF ELEMENT OF CHANCE IS < 5%, RESULT IS CORRECT.

IF P VALUE IS < O.O5 THERE IS DIFFERENCE BETWEEN TWO TECHNIQUES SO NULL HYPOTHESIS IS REJECTED. IF P VALUE IS < O.O5 THERE IS DIFFERENCE BETWEEN TWO TECHNIQUES SO NULL HYPOTHESIS IS REJECTED.

CUT OFF FOR P VALUE

Arbitrary cut-off 0.05 (5% chance of a false +ve conclusion.

If p<0.05 statistically significant- Reject H0, Accept H1

If p>0.05 statistically not-significant- Accept H0, Reject H1

Testing potential harmful interventions ‘α’ value is set below 0.05

LOW P VALUE• If p is very small (<0.001), then the null hypothesis appears not realistic because the difference could hardly ever arise due to chance, when the null hypothesis is true.

P VALUE IS MAXIMUM PROBABILITY OR CHANCE TO COMMIT TYPE 1 ERROR.

BIGGER THE SAMPLE SIZE MORE ACCURATE WILL BE THE DATA

PREVELLANCE OR BURDEN OF DISEASE

QUALITATIVE STUDY

nN = Sample sizeZ = 1.96 (standard normal variate)

P =prevalence (anticipated population proportion)

q = 1- Pd = (permissible error in the estimate of P)

Example:

To estimate the prevalence rate of ascariasis

in a community where it is approximately

known to be 40 percent than the required

sample size to estimate the morbidity

(ascariasis) with 5 percent error with a

probability of 0.05 is calculated as follows:

Ans. 2400 persons are to be examined to estimate the prevalence rate of ascariasis with 5 percent error.

For Quantitative DataThe sample size is calculated by the formula

Where.n = desired sample size.s = standard deviation of observation.e = permissible error in the estimation of mean,t = is the value of ‘t’ at 5 percent level from ‘t tables.

Example. (a) In a community survey to estimate the hemoglobin level, from the data already available if it is known that the mean Hb percent level is about 12 gm percent with a standard deviation of 1.5 gm percent then the sample size required to estimate the Hb level with a permissible error of 0.5 gm percent on either side is obtained as follows. S = 1.5 gm. e = 0.5 gm, t can be taken as 2 (for 1.96: Annexure-III) as it is conventional to use 5 percent level of significance.

In clinical trials usually there will be two groups one experimental and the other control group. In order to estimate the size of the sample for each group, the difference in the response rates of the two groups is to be taken into consideration and the sample size is estimated from the following formula.

Where,n = required sample size for each group.s = pooled SD of the observation of the two groups.d = anticipated smallest difference in the estimates for the two groups.t = is usually taken as “t” at 5 percent level.

WHO SAMPLE SIZE CALCULATOR

SUGGESTED WEBSITE FOR SAMPLE SIZE CALCULATOR

http://www.raosoft.com/samplesize.htmlhttp://www.quantitativeskills.com/sisa/calculations/samsize.htmhttp://www.openepi.com/Menu/OpenEpiMenu.htm

Thank YouKhalid N Haque