General Betting
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If you have a population of 600, how big a sample do you need to get a 95% and a 99% confidence level that you have an accurate answer?
Assume its a heads/tails probability of 0.5 .
Also, how would this change if the probability was 0.75 v 0.25?
Assume its a heads/tails probability of 0.5 .
Also, how would this change if the probability was 0.75 v 0.25?
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that question does not make much (if any) sense.
1 - The population size is irrelevant if your question has a variable sample size 2 - accurate answer for what? 3 - CI can always be calculated - I have a sample of 1 observation I can estimate a confidence interval for the true probability of heads, it's going to be 0%-100%. |
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If the population is 600, I can choose what size sample I want. Its not a "variable sample size". I am looking to find how many people I would need to sample to get a statistically representative/significant guide to the overall population.
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Well if you can chose your sample size then I would say it is variable, in that it can be chosen. Although I was wrong about irrelevance of population.
Try this: http://www.surveysystem.com/sscalc.htm If you want to be 95% confident that the population average lies within +- 5% points of your sample average then you need to sample 234 people. |
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Thanks for the link.
Ok, so 95% of the time, if I sample 234 people, I will get an answer of 112 to 122 , if p=0.5 ? Is that right? |
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Actually, will rephrase this question, just to make absolutely clear.
There is a population of 600, of which I do not know the results for any of them. I wish to take a sample of this population, and get a reliable guide to the overall population with 95% confidence, that I have a result which I can say with 95% confidence is accurately representative of the overall population. What size sample do I need to take to get that? |
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This is exactly what that calculator is answering, so IF you define 'accurate' as + or - 5% then 234 people.
I just pulled +/- 5 out of thin air. |
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Thanks JLivermore.
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np
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Anyone who cannot use apostrophes won't be able to understand the answer.
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m'lud I would like to come to the defence of my learned friend OP. He has contracted the word "whizzes" to "wiz's" - which isn't really an english word, but does follow the phonetic rules of text-speak. So his crime (if it is a crime)is merely to write in the fashion of today's youth and not the greatest of grammar sins, the misused apostrophe.
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