How to determine sample size

By Mangesh Shirpurkar

Question:I want to survey a large group of people. What size should my sample be?Twenty percent? Thirty percent?

Avoid:There is no set percentage.What matters a actual number or size of sample.

Tossing a coin:For five times Average result will skewed widely in one direction.Is not perfectIn long run the average result will be a normal curve. That is evenly split between Heads and Tails

So if you surveyed 20 % of a group of 300 you will get samples of 60 in numbers. Which under represent the population.On the other hand if your population size is 30000, the 20 % of that is 6000 which is to large.

So to avoid this things we should go through the following steps

Steps in selecting a sample size

1. Determine goals2. Determine desired precision of results3. Determine confidence level4. Estimate the degree of Varialibility5. Estimate the Response rate

Determine goals

Know the size of the populationIf your population is small i.e. < = 200, no need of samplingYou will get 0% sampling errorIf your population size is large go through sampling

Decide the methods and design of the sample.Find out what kind of resources you have available.

Determine the desired precision of result

The level of precision is the closeness with which the sample predicts where the true value in the population will lie.The difference between the sample and the real population is called the sampling errorIf the sampling error is + or – 3% means add or subtract 3% from the value in the survey to find out the actual value.This is also called margin of errorThe margin of error depends on balancing accuracy and resources

High levelof precision required larger sample size an higher cost to achieve those samples.

Confidence Level

Determine the confidence levelIt is a risk you want to acceptGenerally 95% means In the sample of 100, 95 samples are in acceptance range and 0.25 rejection range from both the sides

Degree of variability

Variability is the degree to which the concepts being measured80% - 20% Higher the degree of variability uthe larger the sample sizeNote:When the population is extremely heterogeneous i.e. > 90 – 10 a larger sample my be need for better result

Estimate the response rate

When you have come uo with the percentage, you expect to respond then divide the base sample by the percentage of responseE.g. Your estimated response rate is 70% and base sample size 220 then the final sample size would be 220/0.7 = 315

Now you can use this tables

Calculation by formula