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How to determine sample size

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

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How to determine sample size

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  1. How to determine sample size By Mangesh Shirpurkar

  2. 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 perfect In long run the average result will be a normal curve. That is evenly split between Heads and Tails

  3. 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.

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

  5. Steps in selecting a sample size • Determine goals • Determine desired precision of results • Determine confidence level • Estimate the degree of Varialibility • Estimate the Response rate

  6. Determine goals • Know the size of the population • If your population is small i.e. < = 200, no need of sampling • You will get 0% sampling error • If your population size is large go through sampling

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

  8. 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 error • If 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 error • The margin of error depends on balancing accuracy and resources

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

  10. Confidence Level • Determine the confidence level • It is a risk you want to accept • Generally 95% means In the sample of 100, 95 samples are in acceptance range and 0.25 rejection range from both the sides

  11. Degree of variability • Variability is the degree to which the concepts being measured • 80% - 20% • Higher the degree of variability uthe larger the sample size • Note: When the population is extremely heterogeneous i.e. > 90 – 10 a larger sample my be need for better result

  12. 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 response • E.g. Your estimated response rate is 70% and base sample size 220 then the final sample size would be 220/0.7 = 315

  13. Now you can use this tables

  14. Calculation by formula

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