# I am getting a number below zero when caluclating out two standard deviations from the mean. Is this ok?

I am not a math wizard, so please keep your response simple enough. I need to complete a statistics screening exam for a methods course later on today and I am hung up on one topic that came up during the practice test. The data set I got was in reference to the number of homicides that have occurred in a number of cities. This range of the data is 0-5. When I am putting together confidence intervals and calculating out as much as two standard deviations from the mean I am getting low values that are negative. Obviously you can not have a negative number of homicides. When calculating the confidence intervals and out to two standard deviations from the mean should present the low value at ZERO or should I actually present the negative number? For example, if a 95% CI caused the calculation to be -1.5 to 3, would I present that or would I present 0 to 3? Thanks.

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Presumably the "confidence interval" was obtained by assuming approximate normality in a situation where the distribution is far from normal. So the entire calculation is suspect. – André Nicolas Jan 17 '13 at 8:17
Andre is possibly correct about this. You may want to include some of your work in obtaining this result. If on the other hand you are confident about your result, my answer below is correct. – Baby Dragon Jan 17 '13 at 8:21
Are you sure there can't be negative homicides? Maybe some of the cities have zombies in them. – KCd Jan 17 '13 at 10:09
I agree with Andre. I think homicides probably follow a Poisson distribution. – in_wolframAlpha_we_trust Jan 17 '13 at 10:13
The 'less than zero' simply means the distribution isn't a perfect bell curve. Just like a live adult can't have negative height or weight, even if the curve shows that some portion should, the homicide rate can't go negative. – JoeTaxpayer Aug 14 '14 at 1:36