1 / 18

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

graceland
Télécharger la présentation

How to determine sample size

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related