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602 andrew gelman stats-2011-03-06-Assumptions vs. conditions


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Introduction: Jeff Witmer writes: I noticed that you continue the standard practice in statistics of referring to assumptions; e.g. a blog entry on 2/4/11 at 10:54: “Our method, just like any model, relies on assumptions which we have the duty to state and to check.” I’m in the 6th year of a three-year campaign to get statisticians to drop the word “assumptions” and replace it with “conditions.” The problem, as I see it, is that people tend to think that an assumption is something that one assumes, as in “assuming that we have a right triangle…” or “assuming that k is even…” when constructing a mathematical proof. But in statistics we don’t assume things — unless we have to. Instead, we know that, for example, the validity of a t-test depends on normality, which is a condition that can and should be checked. Let’s not call normality an assumption, lest we imply that it is something that can be assumed. Let’s call it a condition. What do you all think?


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1 Jeff Witmer writes: I noticed that you continue the standard practice in statistics of referring to assumptions; e. [sent-1, score-0.615]

2 a blog entry on 2/4/11 at 10:54: “Our method, just like any model, relies on assumptions which we have the duty to state and to check. [sent-3, score-0.874]

3 ” I’m in the 6th year of a three-year campaign to get statisticians to drop the word “assumptions” and replace it with “conditions. [sent-4, score-0.714]

4 ” The problem, as I see it, is that people tend to think that an assumption is something that one assumes, as in “assuming that we have a right triangle…” or “assuming that k is even…” when constructing a mathematical proof. [sent-5, score-0.788]

5 But in statistics we don’t assume things — unless we have to. [sent-6, score-0.343]

6 Instead, we know that, for example, the validity of a t-test depends on normality, which is a condition that can and should be checked. [sent-7, score-0.432]

7 Let’s not call normality an assumption, lest we imply that it is something that can be assumed. [sent-8, score-1.016]


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Introduction: Jeff Witmer writes: I noticed that you continue the standard practice in statistics of referring to assumptions; e.g. a blog entry on 2/4/11 at 10:54: “Our method, just like any model, relies on assumptions which we have the duty to state and to check.” I’m in the 6th year of a three-year campaign to get statisticians to drop the word “assumptions” and replace it with “conditions.” The problem, as I see it, is that people tend to think that an assumption is something that one assumes, as in “assuming that we have a right triangle…” or “assuming that k is even…” when constructing a mathematical proof. But in statistics we don’t assume things — unless we have to. Instead, we know that, for example, the validity of a t-test depends on normality, which is a condition that can and should be checked. Let’s not call normality an assumption, lest we imply that it is something that can be assumed. Let’s call it a condition. What do you all think?

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Introduction: Andy Cooper writes: A link to an article , “Four Assumptions Of Multiple Regression That Researchers Should Always Test”, has been making the rounds on Twitter. Their first rule is “Variables are Normally distributed.” And they seem to be talking about the independent variables – but then later bring in tests on the residuals (while admitting that the normally-distributed error assumption is a weak assumption). I thought we had long-since moved away from transforming our independent variables to make them normally distributed for statistical reasons (as opposed to standardizing them for interpretability, etc.) Am I missing something? I agree that leverage in a influence is important, but normality of the variables? The article is from 2002, so it might be dated, but given the popularity of the tweet, I thought I’d ask your opinion. My response: There’s some useful advice on that page but overall I think the advice was dated even in 2002. In section 3.6 of my book wit

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