On 2016 Jun 17, David C. Norris commented:

Four deficiencies in Dr Kessler's reply may be noted:

• Kessler describes as a "strong statement" my assertion that "a unitary PTSD construct cannot be presumed to support meaningful comparisons of prevalence across environments differing in ambient violence." Aside from his merely technical error (properly speaking, it is such a presumption and not its negation that is logically the 'strong' statement^1§15 ), by labeling my assertion as 'strong' Kessler overlooks its sharp qualification by "unitary" and "meaningful" -- words that point the way to an opportunity for productive critical discourse.

• Kessler asserts that my negating this presumption "dismisses psychiatric epidemiological research on PTSD, which is based [on it]". In so doing, he seems to employ no fewer than 3 logical fallacies² : He advances an argument from authority [of said research programme], and employs the fallacy of the excluded middle to produce a straw man caricature of my point. Far from 'dismissing' such epidemiologic research, my sharply qualified caution indicates how such research may be improved. From a point of view in which the meaning in "meaningful" is to be sought not in the tabulation of p-values but in the critical examination of open concepts³ , a PTSD epidemiologist may readily acknowledge a "unitary" PTSD construct -- ungraded as to severity and undifferentiated as to causation -- as worthy of critical appraisal.

• To my question, unmistakably cast in terms of construct validity, Kessler then responds in terms of criterion validity, ignoring a distinction that was articulated even at the very birth⁴ of construct validity 60 years ago. The phrase "construct validity" appears twice in my Letter; each time, I speak of the "construct validity of the instruments used" relative to "mental health" as (unmistakably) the target construct. Dr Kessler's discussion of "concordance" in para. 3 of his reply is completely irrelevant to this issue.

• When at last, in para. 4 of his reply, Kessler does begin to address a point of construct validity, he does so only from a perspective that is "primarily concerned with the adequacy of tests as measures of a construct rather than with the adequacy of a construct"^5p100 . Indeed, even then, only a convergent validity argument is pursued, with no effort at an exploration of discriminant validity (such as I solicited in the 2nd paragraph of my letter), which might have gone some way toward addressing my criticism.

[1] Popper, Karl. The Logic of Scientific Discovery. 2 edition. New Delhi: Routledge, 2002.

[2] Sagan, Carl. “Chapter 13: The Fine Art of Baloney Detection.” In The Demon-Haunted World: Science as a Candle in the Dark, 1996. See pp. 212-216.

[3] Meehl, Paul E. “Theoretical Risks and Tabular Asterisks: Sir Karl, Sir Ronald, and the Slow Progress of Soft Psychology.” Journal of Consulting and Clinical Psychiatry 46 (1978): 806–34. Full text

[4] CRONBACH LJ, 1955 Full text

[5] CAMPBELL DT, 1959 Full text

On 2016 Jun 17, David C. Norris commented:

Four deficiencies in Dr Kessler's reply may be noted:

• Kessler describes as a "strong statement" my assertion that "a unitary PTSD construct cannot be presumed to support meaningful comparisons of prevalence across environments differing in ambient violence." Aside from his merely technical error (properly speaking, it is such a presumption and not its negation that is logically the 'strong' statement^1§15 ), by labeling my assertion as 'strong' Kessler overlooks its sharp qualification by "unitary" and "meaningful" -- words that point the way to an opportunity for productive critical discourse.

• Kessler asserts that my negating this presumption "dismisses psychiatric epidemiological research on PTSD, which is based [on it]". In so doing, he seems to employ no fewer than 3 logical fallacies² : He advances an argument from authority [of said research programme], and employs the fallacy of the excluded middle to produce a straw man caricature of my point. Far from 'dismissing' such epidemiologic research, my sharply qualified caution indicates how such research may be improved. From a point of view in which the meaning in "meaningful" is to be sought not in the tabulation of p-values but in the critical examination of open concepts³ , a PTSD epidemiologist may readily acknowledge a "unitary" PTSD construct -- ungraded as to severity and undifferentiated as to causation -- as worthy of critical appraisal.

• To my question, unmistakably cast in terms of construct validity, Kessler then responds in terms of criterion validity, ignoring a distinction that was articulated even at the very birth⁴ of construct validity 60 years ago. The phrase "construct validity" appears twice in my Letter; each time, I speak of the "construct validity of the instruments used" relative to "mental health" as (unmistakably) the target construct. Dr Kessler's discussion of "concordance" in para. 3 of his reply is completely irrelevant to this issue.

• When at last, in para. 4 of his reply, Kessler does begin to address a point of construct validity, he does so only from a perspective that is "primarily concerned with the adequacy of tests as measures of a construct rather than with the adequacy of a construct"^5p100 . Indeed, even then, only a convergent validity argument is pursued, with no effort at an exploration of discriminant validity (such as I solicited in the 2nd paragraph of my letter), which might have gone some way toward addressing my criticism.

[1] Popper, Karl. The Logic of Scientific Discovery. 2 edition. New Delhi: Routledge, 2002.

[2] Sagan, Carl. “Chapter 13: The Fine Art of Baloney Detection.” In The Demon-Haunted World: Science as a Candle in the Dark, 1996. See pp. 212-216.

[3] Meehl, Paul E. “Theoretical Risks and Tabular Asterisks: Sir Karl, Sir Ronald, and the Slow Progress of Soft Psychology.” Journal of Consulting and Clinical Psychiatry 46 (1978): 806–34. Full text

[4] CRONBACH LJ, 1955 Full text

[5] CAMPBELL DT, 1959 Full text