The Histogram dialog gives you everything you want to know, and then some.
First, you should convert the image to grayscale. This can eliminate color variations from skewing the results.
First figure out what particular pixel you plan to sample, and what range of similar pixels is appropriate to include in the sampling. Set the Wand tool appropriately.
You click on a particular gray area with the wand, then pull up the Image > Histogram dialog to get the statistics.
Move your mouse very slowly over the lines and you'll see that each vertical row of pixels represents a single level in the selected group of pixels. Theoretically you could assign specific "ailment" to a certain level or groups of levels.
1. Mean: the average brightness level for all the pixels in the selection
2. Std Dv: the average difference between the highest and lowest levels
3. Median: where the exact "middle" of the selected range occurs
4. Pixels: how many pixels are actually included in that selection
5. Level: gives you the numeric "level" of any line you hold the cursor over
6. Count: gives you the number of pixels in the selection which match the selected level
7. Percentile: how many pixels possess a value darker than the selected one.
From that information you could (again, theoretically) build some fairly sophisticated analysis criteria for analyzing objects in your photographs. But there's so much more information to be obtained there, as well.
Suppose you've designated which specific levels or ranges of levels represent an ailment or condition or feature in the suspected object, mass for example.
Now you can use the Select > By Color command and select those levels (colors) which stand for the specific data searched for and the Histogram will give you the statistics of the selection. You could fine-tune that to a single value if you wish to, then run the Histogram for the whole file.
Once you start playing around with the feature and doing 'what-if' scenarios it becomes quite interesting.
From the book "The Complete Ghost Hunter" by Polston and Carter