# Estimating the number of cases (sample size)

#### unknown

##### New Member
Hi there!

I've a perhaps simple question: how can I estimate the number of measurements (cases) to obtain a accurate result?

Here an example:
I'ld like to know the average body length of a trout population in 100 ponds. The question is now, how many trouts of each pond do I need to measure. Indeed, measuring 100 per pond is better than 10. But is there somewhat like a statistical method to estimate the needed sample size per pond? I found some blogs dealing with reliability analysis, but they use it only for questionaries. Is there something similar for my case?

Thanks!

#### Dason

Depends on what you want to do. How do you define accurate? Are you wanting to do some sort of statistical test or do you just want a fairly short confidence interval around your estimate? You need to be specific with what you are interested in.

#### kiton

##### New Member
I suggest you use a tool called G*Power (http://www.gpower.hhu.de/) that would allow you to determine the "required" sample size based on the desired effect size, alpha error probability, and power.

#### unknown

##### New Member
Hi and thanks!

Ok, I guess I formulated my question in a confusing context.. I try to estimate how many fish I need to measure to get an representative result.
In other words: what is my min. sample size to obtain a mean and standard deviation that is close (maybe 10% deviation) to the real values of the populations?

#### Dason

To actually be able to do this you need to be able to talk exact numbers. "Close" isn't something that you can plug into a formula. Do you want to be able to estimate the mean within 3 inches? What is your tangible goal. Those are the kinds of things you need to be able to answer.

#### unknown

##### New Member
Hi Dason,

thanks again!
I guess I need to get my hands dirty with this topic before I come with further questions

#### kiton

##### New Member
Just get G*Power -- it does exactly what you ask for (taking in account Dason's comments, in a way).