Accepted Solutions
Stephanie,
I saw zeros in your data, but no missing values. The error I received was "insufficient error degrees of freedom." With 2 df for each treatment group that gives you 8 df for each time period (repeat). So I expected only 4 time periods would be allowed for the test. To be fair or to show what is seen in the data, use 4, 12, 72 and 120 it will show the sphericity issue.
Actually, your dataset is quite interesting. I don't think you need a statistical test to note an important difference in the materials' results. The data exhibits a growth pattern.
Using the wide data set you had attached, create the covariance matrix using the Multivariate analysis, and select Covariance. You will see that the covariance structure (and correlation) are quite different for early periods vs. later periods where the measured values are not changing very much. Again you should look at the data and covariance you can see that sphericity rule is violated without a test. In my opinion, a p-value has no value here.
So the next step is to chose a different test. You should at least model a time series component for your repeated measures. (See suggestion below.)
If you want to quantify the difference, I suggest you model the growth for each treatment group. The pictures above display the measured values, the results of Fit Curves using a Weibull Growth curve and the perforfmance. Note the Weibull Growth model is not in JMP13, but is available in JMP14. The predicted values were saved to the data table and plotted vs. actual with the line y=x. The scripts for both are saved to the stacked version of your table (attached). The Fit Curves script might not work, depending upon your version of JMP.
You should investigate the instability of the BF treatment. Is there something in the material, the environment like the location of the box or temperature that might be affecting the precision of your readings for the phosphorous concentrations.
Stephanie,
I saw zeros in your data, but no missing values. The error I received was "insufficient error degrees of freedom." With 2 df for each treatment group that gives you 8 df for each time period (repeat). So I expected only 4 time periods would be allowed for the test. To be fair or to show what is seen in the data, use 4, 12, 72 and 120 it will show the sphericity issue.
Actually, your dataset is quite interesting. I don't think you need a statistical test to note an important difference in the materials' results. The data exhibits a growth pattern.
Using the wide data set you had attached, create the covariance matrix using the Multivariate analysis, and select Covariance. You will see that the covariance structure (and correlation) are quite different for early periods vs. later periods where the measured values are not changing very much. Again you should look at the data and covariance you can see that sphericity rule is violated without a test. In my opinion, a p-value has no value here.
So the next step is to chose a different test. You should at least model a time series component for your repeated measures. (See suggestion below.)
If you want to quantify the difference, I suggest you model the growth for each treatment group. The pictures above display the measured values, the results of Fit Curves using a Weibull Growth curve and the perforfmance. Note the Weibull Growth model is not in JMP13, but is available in JMP14. The predicted values were saved to the data table and plotted vs. actual with the line y=x. The scripts for both are saved to the stacked version of your table (attached). The Fit Curves script might not work, depending upon your version of JMP.
You should investigate the instability of the BF treatment. Is there something in the material, the environment like the location of the box or temperature that might be affecting the precision of your readings for the phosphorous concentrations.
