Question

Caterpillar nitrogen assimilation and body mass. we explored the relationship between nitrogen assimilation and body mass (both on log scales) for data on a sample of caterpillars in Caterpillars. The Instar variable in the data codes different stages (1 to 5) of caterpillar development.
a. Fit a model to predict log nitrogen assimilation (LogNassim) using log body mass (LogMass). Report the value of for this model.
b. Fit a model to predict LogNassim using appropriate indicators for the categories of Instar. Report the value of for this model and compare it to the model based on LogMass.
c. Give an interpretation (in context) for the first two coefficients of the fitted model in (b).
d. Fit a model to predict LogNassim using LogMass and appropriate indicators for Instar. Report the value of and compare it to the earlier models.
e. Is the LogMass variable really needed in the model of part (d). Does the linear trend appear to be better when the caterpillars are in a free growth period? (Again, we are not ready to fit more complicated models, but we are looking at the plot for linear trend in the two groups.)

Answer 1

A. Produce a scatterplot for predicting nitrogen assimilation (Nassim) based on Mass. Comment on any patterns.

b B.Produce a similar plot using the log (base 10) transformed variables, LogNassim versus Log Mass. Again, comment on any patterns.

C. Would you prefer the plot in part (a) or part (b) to predict the nitrogen assimilation of caterpillars with a linear model? Fit a linear regression model for the plot you chose and write down the prediction equation.