- #1

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1. Homework Statement

1. Homework Statement

Here is the theorem (begins end of page 1), it is not my course but I have almost the same demonstration : http://math.arizona.edu/~jwatkins/sufficiency.pdf

Screenshot of it:

## Homework Equations

Could someone please explain me how to justify the first equality of that step?

## The Attempt at a Solution

I think a possible justification is because the sample is a sufficient statistic but it feels like it's not enough/not the right justification