The concept of Null Hypothesis can be confusing for many of us. It is a statement of nonexistence, For example:
"There is no (Statistically Significant) difference between the Means of these two Populations."
And we normally think in terms of what exists, not what doesn't. It would be more natural for most of us to start by asking a question instead:
"Is there a (Statistically Significant) difference between the Means of these two Populations?"
Then, we could rephrase it as a Negative Statement to produce a Null Hypothesis, as shown below.
Sums of Squares is an important concept in Variation, playing a major role in ANOVA and Regression. 4 Keys to Understanding and plenty of graphics help to give the viewer an good conceptual understanding.
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Andrew A. (Andy) Jawlik is the author of the book, Statistics from A to Z -- Confusing Concepts Clarified, published by Wiley.