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I am running a Chisquare test using CHIDIST function in Excel. My observed and expected values are not identical however i get result 0 (zero). There are 75 observed values so I am using 74 freedom points. Not sure what I am doing wrong, any assistance would be appreciated.

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The description of the CHIDIST Excel function says:

Returns the right-tailed probability of the chi-squared distribution. The $\chi^2$ distribution is associated with a $\chi^2$ test. Use the $\chi^2$ test to compare observed and expected values.

This means that the p-value displayed by the function will be lower when the observed valuer are far away from the expected ones; and the p-value will be $1$ if the observed values match exactly the expected values. Therefore, since you are getting a very small p-value (displayed as zero due to the rounding done by Excel), I deduct that your data is distinct from the expected distribution.

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  • $\begingroup$ Thanks for the reply. I did the chi square test as sum of (observed-expected)^2/expected and the number i got is quite big - around 2,700,000 which I am not sure if makes sense. Regarding the rounding I don't know how small the number would be for excel or numbers in mac to not show it as I tried upto 30decimals and still result is zero. Is there any way to see the actual number? I $\endgroup$ – DimitrisM Oct 1 '19 at 15:34
  • $\begingroup$ @DimitrisM : 2.7 million is a lot! Are you sure you haven't made a mistake? For 74 degrees of freedom, a p-value of $0.001$ would be attained for $\chi^2$ around $117$; and a p-value of $10^{-30}$ would be attained for $\chi^2$ around $310$. $\endgroup$ – Ertxiem - reinstate Monica Oct 1 '19 at 22:25
  • $\begingroup$ It does seem a lot to me also but not sure where I got it wrong. I calculate x square by summing the squared differences of observed minus expected values and dividing the result by expected value, for each row. $\endgroup$ – DimitrisM Oct 2 '19 at 14:24
  • $\begingroup$ The link to the data I am using is this if you want to have a look. The observed/estimated values are flu count incidents per US county. github.com/DimitrisMertikas/Chisquaretesting $\endgroup$ – DimitrisM Oct 2 '19 at 15:14
  • $\begingroup$ @DimitrisM: I've checked your data and I got $\chi^2 = 2\ 189 \ 770.58$ for $df=74$. So, It's natural to have such a low p-value. Looking at the data, you see counties with an incidence of nearly $43\%$, and others with incidences of $0.01\%$, thus, the p-value is telling that there are significant differences between counties, which is according to what is observed. $\endgroup$ – Ertxiem - reinstate Monica Dec 17 '19 at 22:28

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