Comparing lettuce shelf life with and without fertilizer during production

Question

Hi there,

I suspect what I'm trying to do is very simple, but I'm not sure the best way to go about it.

I recent ran an experiment comparing shelf-life of lettuce plants grown either with added fertilizer or without added fertilizer. Every day, the lettuce was ranked on plant quality, with 5 being greatest, and 1 being the worst. I can plot this information on a graph of plant quality by days after harvest and see a curve for the fertilized plants, and a curve for the non-fertilized plants. How do I determine if these curves significantly differ from each other? Thank you in advance.

cjd80 · Answer

Thank you Dr. Bailey! This makes sense and was easy to execute.

Mark_Bailey · Answer

You can use logistic regression for this analysis. The rating is the response (Y role). It can be character or numeric data but it should have the ordinal modeling type. Fertilizer is a binary nominal variable. Time is a continuous variable.

Select Analyze > Fit Model. Select rating and click Y. Select fertilizer and time, click Macros, and select Full Factorial. Now click Run. The cross term tests for an interaction effect. Essentially the question is if the effect of time depends on the level of fertilizer.