# Sample size calculation--population is known

#### bhardwajsaurabh

##### New Member
How to check the sample size with below values
1)Average population:0.1 mil
2)CL-95%
3)MOE-0.5
4)SD-18300

please suggest the sample size calculation formula ,when population is known

#### obh

##### Active Member
Hi Saurabh,

Do you know what is the formula for the confidence interval when you know SD?
What part of this formula is Margin Of Error?

#### bhardwajsaurabh

##### New Member
Hi Saurabh,

Do you know what is the formula for the confidence interval when you know SD?
What part of this formula is Margin Of Error?
Hi,

I have monthly population via which i have checked SD.

can you help calculate the sample ,when population is 0.1mil /CL:95%/CI-+-5

Regards,
Saurabh Bhardwaj

#### bhardwajsaurabh

##### New Member
Want to know the sample size for survey.

Average:1,29,517
STD:20,914
N=12
CL-1.96

#### obh

##### Active Member
Hi,

You are really doing a mess with the numbers... you change the numbers all the time

What is average? :0.1 , 120000, 129,517 1,290,517 ? you should put the comma every 3 digits.
what is the STD: 18300 20,914?

With a confidence level of 0.95 => α=0.05 => 1-0.05/2=0.975 => Z_0.975=1.96

You probably mean that the MOE=5% not 5. that is mean a percentage of the average.
To get an accuracy of +-5 with a standard deviation of 18300 you need a huge sample size ... 5% is a different story ...

Also is the population finite?

#### bhardwajsaurabh

##### New Member
Hi,

You are really doing a mess with the numbers... you change the numbers all the time

What is average? :0.1 , 120000, 129,517 1,290,517 ? you should put the comma every 3 digits.
what is the STD: 18300 20,914?

With a confidence level of 0.95 => α=0.05 => 1-0.05/2=0.975 => Z_0.975=1.96

You probably mean that the MOE=5% not 5. that is mean a percentage of the average.
To get an accuracy of +-5 with a standard deviation of 18300 you need a huge sample size ... 5% is a different story ...

Also is the population finite?
****
Thanks for your patience..yes the population is finite.

Average:129,517
STD:209,14
MOE:-5%
N=12
CL-1.96(95%)

#### obh

##### Active Member
N=12 say the entire population is 12 ???

#### obh

##### Active Member
Hi Saurabh,

I thought the question is what is the required sample size???

commonly used:
N - population sample size, for example, id there are 10,000,000 voters in the country and you sampled 5000: N=10,000,000 n=5000.

#### bhardwajsaurabh

##### New Member
Hi Saurabh,

I thought the question is what is the required sample size???

commonly used:
N - population sample size, for example, id there are 10,000,000 voters in the country and you sampled 5000: N=10,000,000 n=5000.
Hi,

Yes my question is the desired sample size ,when average population is 129,517

#### obh

##### Active Member
Can you please give more details?
What do you mean average population? The average of the population or the sample size of the population?

#### bhardwajsaurabh

##### New Member
Can you please give more details?
What do you mean average population? The average of the population or the sample size of the population?
Hi,

Take it as population of a city is 129,517,how many sample should i survey to get opinion poll.

#### obh

##### Active Member
Hi Saurabh,

Slowly slowly I will understand your question ...
It is better to support all the data at the beginning ... (but shortly...)

You should probably use the expected proportion of the thing you want to check., not the STD.
What is the STD you mention? STD of what?
http://www.statskingdom.com/50_ci_sample_size.html

#### bhardwajsaurabh

##### New Member
So what was the problem ...?
Dear,

Let me explain you the entire scenario.

I am sharing dummy data below:

In a call center we receive approx 0.1 million calls per month.we want to conduct a survey to gauge the c-sat.

Now i want to know,how many customers should i touch in a month (desired sample) for the survey.

Mean on 12 months call volume is 0.12 million,while SD of call volume in past 12 months is 18,000.

#### obh

##### Active Member
Okay, so the goal is to survey...

If you don't know the expect proportion you should take the worse case: Proportion=0.5
population=129,517
confidence level=0.95
MOE=0.05

sample size=385

This says that if you get the result that 0.5 (50%) of the sample prefer the green color.
The confidence interval will be 0.5±0.05 or 50% ± 5%.