I have a dataset consisting of body weight and corresponding age for a bunch of healthy subjects (grey triangles below). I fit a nonlinear function to this data and graphed a 95% prediction interval. I then graphed the body weights and ages of some experimental subjects (colored circles).
One of the experimental subjects, the light blue circle, falls outside the 95% prediction interval from the control data. Is there any way to describe this one relationship statistically? E.g. significance testing? I am not interested in comparing the group means of the control vs. experimental subjects; I am only interested in the light blue subject.
Alternatively, I have 13 healthy controls that were the same age as the light blue subject. If I convert their body weights to Z-scores, the Z-score of the light blue subject is ~3.5. Can I just convert it to a p-score (~0.001), or would I be making an invalid statistical assumption?
I really appreciate any help - it's been years since I had even basic statistics.
To clarify, the grey triangles on this graph are healthy controls. The nonlinear regression and 95% prediction interval were created only from this control data.
The colored circles are experimental subjects. The light blue circle corresponds to a subject with numerous health problems; I'm wondering if there's a way to statistically describe this subject's relationship to the control data, or if I am limited to qualitative comparisons.