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14 MAR 2013

How many responses do I need to conduct online research

Are you introducing a new product on the market? Work on client segmentation? Or maybe you plan important changes for existing products? In all those tasks online survey research can be more than helpful.

In ideal world you would ask each person (e.g. client) in your population about his/her opinion, preferences or behavior. As your target group may be very large and dispersed you may not be able to do so. Luckily it is not necessary. You can randomly select from a population certain number of people and conduct survey on a selected sample.

Sounds good, but how many people do I need to start my online research? We will help you to answer this question.

Estimate population size

If you don't know how to start use Internet or go to the statistical office web site. In each county there is statistical office that can provide you (usually free of charge) with the general information about economy and population.

Decide how accurate survey results you need

Because you won't ask every person in your target population about his/her opinion, your survey results will contain some statistical error. You need to decide how big error you are willing to accept. Most of researchers decide for 5% error, but you can use any value from range 1-10%. Remember, the smaller error the more accurate results you will get.

Calculate sample size

Now you can calculate your sample size. To make this task easier we created table for three margin errors.

  margin error
population size 1% 5% 10%
500 475 217 81
1.000 906 278 88
5.000 3288 357 94
10.000 4899 370 95
50.000 8057 381 96
100.000 8762 383 96
500.000 9423 384 96
1.000.000 9513 384 96
5.000.000 9586 384 96

Please treat these values as a guideline. You can round up your sample size to the nearest tens, hundreds or thousands.

Do you wander how we have calculated values in a table above. Below you will find a formula (for finite population) and explanations that will allow you to calculate sample size for any population.

Formula

n = P*(1-P) / (e^2/z^2) / (P*(1-P)/N)

Explanations

n - sample size, in other words how many responses you need to collect
N - size of the population
P - estimated proportion in the population (if you don't know what to enter use 50%)
e - margin error (in the table above those are values 1%, 5% and 10%)
Z - Z value, which is connected to the selected confidence level. Most of researchers use 95% confidence level. Z value for 95% is equal to 1.96.

Estimate response rate

Remember that your sample are people that actually completed surveys, not people that received invitation to participate in a survey. Those numbers can be significantly different. Therefore it is important to estimate response rate and take it into account as you plan your survey.

Response rate is a relation between number of people that completed survey and number of send invitations. Good result in online survey research is 20-30%.

NOTE. If you are using mysurveylab.com survey panel, you will pay only for delivered responses. This mean that we will take care for proper response rate and collection of requested number of response, so you can relax.


Estimate how many people do you need for your market research

Now you have all date to calculate how many people you need for your survey research. You need to divide sample size by response rate. Number you will get is a number of survey invitations you need to send in order to collect proper number of responses.

sample size / response rate

Example. If your target population counts 500.000 people and you are willing to accept 5% statistical error, than for your research you will need  384 / 20% = 1920 people. You can round up this number to 2000 people.

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