I'm designing an experiment to evaluate reagents used to develop fingerprints. There will be a variety of objects with different surfaces, 2 different reagents, 3 different fingerprint donors (prints deposited intentionally on the object), and 3 human graders. Photos will be taken of the developed prints on the objects for the grader to visually evaluate.
I've created a 5 level visual standard, similar to a Likert scale, to use for grading both the print development and the background development. High contrast (strong print development with minimal background development) is the ideal situation. For the visual standard, there are separate scales (0-4) for the print and the background, with visual representations of each level for both.
From what I understand, the data generated from this experiment would be discrete. I'm unsure whether it would be considered ordinal, interval, or ratio. It seems like it would have more information than just a simple ranking (for ordinal) because of using the visual standard. I think it makes sense to say there is an “absolute zero” because there will be examples of no print development and no background development. Does this mean the data would be considered a ratio subtype? Or is there another datatype that I’m not aware of that is more appropriate?
I’m also unsure of how to treat the dual grading scale results (for the print and the background). This doesn’t seem like the classic “before” and “after” medical study scenario of paired data. But I’m not sure if the dual grades would be considered “paired” in a different sense. Perhaps I need to make a single “combined” grade using the ratings for the print and the background, but I’d like to keep them independent, if possible.
The most important question to answer for the experiment will be whether there is a statistically significant difference between the two reagents (which reagent produces the highest contrast prints). I’m assuming the data will probably not be normally distributed, so I’ll need to use a nonparametric test. I don’t have a statistical software package to use, but I do have Excel.
I’d appreciate hearing recommendations for the type of hypothesis test to use (with the consideration of doing the calculations with Excel), a rough estimate of how many replicates I’d probably need, how to interpret the results of the hypothesis test , and anything else that is important to consider to draw meaningful conclusions.