# could you please provide me "Watson"s" formula that helped to...

could you please provide me "Watson"s" formula that helped to determine sample size out of a given population.

Outside United States

## 1 Response

I am not sure who "Watson" is, but determining sample sizes typically involves the following steps:

1) What research and null hypotheses will be tested using this sample?

2) What is the expected difference in responses between groups? If a current therapy/intervention has a cure rate of 40% and your new therapy/intervention has a proposed cure rate of 60%, then the difference in improvement between the therapies is 20%.Another way to address this is to specify the effect size that is of *scientific interest*. With a large enough sample, every effect is statistically significant. Determine an effect that has the most “real world” significance.

3) What is the standard error/standard deviation in measurement? This information is usually obtained from pilot studies or historical data that has a similar outcome. If you have the standard error, you can calculate the standard deviation (and vice versa).

4) What is the level of statistical significance? This is the probability of *falsely *concluding the therapies/interventions significantly differ (typically expressed as 0.05).

5) What is the intended power calculation? This is the probability of *correctly *concluding therapies/interventions significantly differ (typically expressed as 0.8 or 0.9).

As a general rule of thumb, a random sample of over 384 individuals is enough to represent a population of 10,000.

Additional resources:

http://www.praccreditation.org/secure/documents/coachHO16.pdf

http://www.surveysystem.com/sscalc.htm

http://bphc.hrsa.gov/policiesregulations/performancemeasures/patientsurvey/calculating.html