Solved – Interpreting a negative confidence limit for a proportion

confidence intervalsampling

The margin of error is driven by the size of the sample.

In a consultant's report (which is confidential at this stage), they collected responses from 10 store managers (out of a total of 200 store managers i.e. target population) and went on to make statements such as "only 20% of the respondents were happy with the sales performance of their stores".

The margin of error in this case is around 32%. Assuming a confidence level of 95%, the true response is between -12 and 52%.

How does one interpret this result (especially the negative part)?

It is wise to use margin of error for small sample sizes (e.g. sample of 15 out of a target population of 60)?

Best Answer

Because it is not possible to have a percentage less than zero, the first interpretation is the response is that between 0% and 52% were happy. Just substitute 0 for the negative percentage.

The bigger question is why they are reporting results with such large margins of error. As a client, knowing that between 0% and 52% of respondents were happy is a pretty meaningless result. With such a small sample size, they are going to get those large margins of error for every single question in the survey. Ten people is just too small a sample size to get robust estimates.

For me, the bigger question is why they didn't use a qualitative method for this work.

Update based on comment below: increasing the margin of error to decrease the sample size is not a good way forward as you have seen with the margins of error you currently have. With a population of 60, you need to sample the entire population, i.e. undertaken a census, in order to get acceptably low margins of error. As for any survey, non-response bias will be a concern.

Recommendation: for the population of 60 either

  1. undertake a census if you want the results to be analysed statistically, or
  2. pick some key individuals in the population on the basis of their known attitudes or views, use qualitative interviews, and report themes (don't do any statistics, not even counts). For 10 people, qualitative interviewing is just as quick as doing a survey anyway.
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