Add guidance against flippant appeals to obviousness

chapters/citations-evidence.qmd

@@ -72,6 +72,115 @@ However,
 when in doubt,
 provide a citation—over-citing is preferable to under-citing.
 
+## Do not substitute flippancy for support
+
+Some phrases assert that a claim needs no support
+instead of supplying any:
+
+- "it is obvious that"
+- "clearly, it follows that"
+- "it should be familiar to you that"
+- "the familiar formula"
+- "of course"
+- "as everyone knows"
+
+The problem is the appeal, not the individual words.
+"Clearly" is fine as an adverb of manner
+("the methods section clearly describes the sampling frame"),
+and "of course" is fine when it flags an assumption
+you have already established.
+Each becomes a problem only when it stands in
+for an argument you have not made.
+
+These phrases do no work for the reader.
+A reader who already knows the result gains nothing
+from being told that it is familiar,
+and a reader who does not
+is told that the gap is their own fault,
+without being given any way to close it.
+They also cost the writer something.
+Working a step out is how you find out whether it holds;
+calling it obvious settles the question by assertion,
+and leaves any error in the step where it is.
+So if you cannot supply a derivation or a citation for a step,
+treat that as a reason to work the step out,
+not as a reason to call it obvious.
+
+Replace such phrases with one of the following:
+
+- **A direct derivation.**
+  Show the intermediate steps,
+  or state which identity or algebraic manipulation takes you from one line to the next.
+- **A specific citation.**
+  Name the source that establishes the result,
+  and give a chapter, section, page, or equation number
+  whenever the source is long enough
+  that a bare citation would leave the reader searching.
+- **A cross-reference.**
+  When you established the result earlier in the same document,
+  point to it by number
+  (see [Defining terms clearly](defining-terms.qmd)
+  for the Quarto cross-reference syntax).
+
+Deleting the phrase and keeping the claim
+is also usually an improvement:
+"the log-likelihood is a sum of three terms"
+makes the same claim about the log-likelihood as
+"the log-likelihood is obviously a sum of three terms",
+and it invites the reader to check the claim
+rather than to defer to it.
+
+:::{#exm-flippant-derivation}
+
+## Replacing an appeal to familiarity
+
+> ❌ Substituting the normal density into the likelihood
+> gives the familiar three-term formula.
+>
+> ✅ For an independent sample $x_1, \ldots, x_n$
+> from a normal distribution with mean $\mu$ and variance $\sigma^2$,
+> substituting the normal density into the likelihood
+> and taking logarithms gives
+> $$
+> \ell(\mu, \sigma^2) =
+> -\frac{n}{2}\log(2\pi)
+> - \frac{n}{2}\log\left(\sigma^2\right)
+> - \frac{1}{2\sigma^2}\sum_{i=1}^{n}\left(x_i - \mu\right)^2.
+> $$
+> The three terms are,
+> in order:
+>
+> - a constant that does not involve the parameters;
+> - a term that depends on $\sigma^2$ but not on $\mu$;
+> - the sum of squared deviations of the data from $\mu$,
+>   divided by $-2\sigma^2$.
+
+The second version gives the reader the formula and where it came from,
+so they can check the algebra themselves.
+The first version asks them to take the writer's word for it,
+and offers no help to anyone who has not seen the result before.
+
+:::
+
+### "Trivial" and "trivially"
+
+"Trivial" and "trivially" usually work the same way:
+they announce that a step is too small to write down,
+which relieves the writer of writing it down.
+Treat them as a prompt to check the step,
+and to write it out if it takes only a line.
+
+The word has one defensible use,
+which is describing how a general expression reduces to a simpler one
+in a special case.
+For example,
+a weighted average reduces to an ordinary average
+when all of the weights are equal.
+Even in that use,
+"minimal case" or "degenerate case"
+names the situation more precisely,
+and avoids the dismissive tone that "trivial" carries.
+
 ## What makes a citation relevant
 
 A relevant citation is one that actually supports the specific claim you are making.
@@ -186,7 +295,8 @@ To effectively support your claims:
 ## Common citation errors to avoid
 
 - **Citation needed**: Making claims without any supporting citation or evidence
-- **Vague attribution**: Using phrases like "studies have shown" without citing specific studies  
+- **Vague attribution**: Using phrases like "studies have shown" without citing specific studies
+- **Appeal to obviousness**: Using "obviously", "clearly", or "as everyone knows" in place of a derivation or a citation
 - **Circular citation**: Citing a paper that doesn't contain the claimed information but cites another paper that does (cite the original source)
 - **Citation padding**: Adding citations that don't actually support your claims just to appear well-referenced
 - **Selective citation**: Only citing work that supports your position while ignoring contradictory evidence
@@ -234,5 +344,5 @@ understand the foundation of your arguments,
 and locate resources for further learning.
 Always ask yourself:
 "How does my reader know this is true?"
-If the answer isn't obvious from your text,
+If your text does not answer that question,
 add a citation or present direct evidence.