Question about 0.5 pseudo-count

[godotgildor]

Looking at the code here:

Enrich2/enrich2/selection.py

Lines 533 to 540 in bb31cfd

	shared_counts = (
	self.store.select(
	"/main/{}/counts".format(label), "columns in ['c_0', c_last]"
	)
	.sum(axis="index")
	.values
	+ 0.5
	)

I notice that for the denominator of the enrichment ratio (shared_counts), the code sums all the values for c_0 and c_last and then adds a single pseudo-count of 0.5. Later, for the numerator, the code adds a pseudo-count of 0.5 to each count in the numerator.

Wouldn't this have the effect of potentially skewing the ratios so that they wouldn't sum to 1? For instance, let's say c_0 = [1, 3, 1, 2]

Then shared_counts = (1 + 3 + 1 + 2) + 0.5 = 7.5

And then the ratios would be:
1.5/7.5 = 0.2
3.5/7.5 = 0.467
1.5/7.5 = 0.2
2.5/7.5 = 0.333

and the sum of the ratios = 1.2 instead of 1.

I would have thought that for shared_counts the code would have added the 0.5 pseudo count prior to the sum (or alternatively added a pseudo_count = 0.5*len(c_0)) ?

I may be misreading things though.