Hello

I am doing a project on count data using SPSS software. I have analysed the number of calls to a call centre over a 18 month period. It is clear that January shows the highest number of calls. I applied the one way anova test and it showed that there was a significant difference between means. I then carried out a post hoc (LSD) analysis. I looked at the multi-comparisons table and the I-J column (mean differences) for January and it showed some minus numbers when compared to some of the other months. How is it possible that Januarys means can be lower to that of other months when it showed a higher call count? Please could someone explain this to me!

Thank you

Uugh, I hate stats.

Exactly what are you taking the mean of? The number of calls per day? If the number of calls is higher in January than in other months, than January would have a higher mean. It's possible that when you took the mean difference you subtracted the mean of January from the mean of some other month, that would give you a negative mean difference.

Thanks for your help. I used statistics SPSS software to calculate the mean differences and used a test called one-way ANOVA. I am not sure how it calculated the mean but I entered the number of calls made each day between October 2007 and March 2009 and it produced a graph with 2 large peaks, one in January and one in May. January clearly had the highest number of calls so I don't understand why its showing a higher peak in May.A negative mean difference implies that the mean in January was lower, it makes no sense!

If memory serves, an ANOVA test tests the variance, a couple of days of really heavy calling in may and a generally high call rate in January would give may a higher variance. I would need to see the data to give you more help. Maybe if you make a PDF and post it, I'll give you what help I can. My degree is in mathematical logic, so stats really isn't my forte, but I'll do my best.