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. 2015 Oct 23;10(10):e0141606.
doi: 10.1371/journal.pone.0141606. eCollection 2015.

Single-Item Measurement of Suicidal Behaviors: Validity and Consequences of Misclassification

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Single-Item Measurement of Suicidal Behaviors: Validity and Consequences of Misclassification

Alexander J Millner et al. PLoS One. .
Free PMC article


Suicide is a leading cause of death worldwide. Although research has made strides in better defining suicidal behaviors, there has been less focus on accurate measurement. Currently, the widespread use of self-report, single-item questions to assess suicide ideation, plans and attempts may contribute to measurement problems and misclassification. We examined the validity of single-item measurement and the potential for statistical errors. Over 1,500 participants completed an online survey containing single-item questions regarding a history of suicidal behaviors, followed by questions with more precise language, multiple response options and narrative responses to examine the validity of single-item questions. We also conducted simulations to test whether common statistical tests are robust against the degree of misclassification produced by the use of single-items. We found that 11.3% of participants that endorsed a single-item suicide attempt measure engaged in behavior that would not meet the standard definition of a suicide attempt. Similarly, 8.8% of those who endorsed a single-item measure of suicide ideation endorsed thoughts that would not meet standard definitions of suicide ideation. Statistical simulations revealed that this level of misclassification substantially decreases statistical power and increases the likelihood of false conclusions from statistical tests. Providing a wider range of response options for each item reduced the misclassification rate by approximately half. Overall, the use of single-item, self-report questions to assess the presence of suicidal behaviors leads to misclassification, increasing the likelihood of statistical decision errors. Improving the measurement of suicidal behaviors is critical to increase understanding and prevention of suicide.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.


Fig 1
Fig 1. A flow chart of the online survey.
Gate questions are single-item questions for three suicidal outcomes. Suicide ideation follow-up questions include eight different thoughts and suicide action follow-up questions include five different actions. Participants that denied any suicidal thoughts or behaviors in the first three sections skipped to the end of the survey. Participants that endorsed any suicidal thought or behavior completed several follow-up questions regarding planning behaviors and narrative descriptions of their behavior.
Fig 2
Fig 2. A visual example of DV-misclassification relationships and the results of the statistical simulations.
(A) Model data of a “true effect" (B) Model data of a “true null effect.” The red and green arrows in (A) and (B) represent data points that will be misclassified due to participants inaccurately categorizing their suicidal behavior. (A) and (C) show an example of when the misclassification is random (i.e., dependent variable (DV)-misclassification correlation is zero). Because true attempters tend to be higher on the DV, true attempters randomly misclassified into the non-attempters group will tend to increase the group mean of non-attempters (e.g., from 39.57 in (A) to 42.17 in (C)). Likewise, in the example, true non-attempters tend to be lower on the DV and therefore, true non-attempters randomly misclassified into the attempters group will reduce the group mean of attempters (e.g., from 51.63 (A) to 44.03 in (C)). Thus, when there is a true difference between the groups at the population level, random misclassification will cause the two group means to move closer together reducing the power to detect the true effect. (B) and (D) shows an example of when there is a pattern to the misclassification (the DV-misclassification correlation is approximately r = 0.20). In this example, attempters higher on the DV tend to misclassify their behavior, causing an increase in the mean of non-attempters (e.g., from 43.66 in (B) to 52.56 in (D)). Non-attempters that are lower on the DV tend to misclassify their behavior, causing a decrease in the mean of attempters (e.g., from 43.76 in (B) to 34.83 in (D)). Thus, during a “true null effect” when the DV-misclassification correlation is present, misclassification causes the group means to shift further apart, increasing the probability of finding a false significant difference. (E) The outcome of the statistical simulation for data with a “true effect” across different effect sizes (varying sample sizes to maintain statistical power at 0.95). Without a DV-misclassification correlation, power to detect the true effect is reduced by approximately 15% across all effect sizes. Power decreases as the DV-misclassification correlation strengthens in the opposite direction of the true effect, increasing the chance of a false negative result (i.e., Type II error). This reduction in power is greater when the effect size is small and the sample size is large compared to when there are larger effect sizes and smaller sample sizes. (F) The “true null effect” across the same sample sizes as in the true effect simulation (alpha = 0.05). When misclassification is random for a “true null effect,” it does not affect the false positive error rate. As the DV-misclassification correlation strengthens in either direction, chances of a false positive increase (i.e., Type I error). Compared to larger sample sizes, chance of a false positive is greatest when there is a smaller sample size.
Fig 3
Fig 3. Characteristics of suicide planning among participants who attempted suicide and either endorsed a single-item plan or did not.
For those that endorsed a single-item lifetime suicide plan (blue) and for those that denied a single-item lifetime suicide plan (red), the distributions of (A) the amount of time between thinking of the method and attempting suicide, (B) the decision to attempt and attempting suicide and (C) thinking of place to attempt suicide and attempting suicide and (D) the percentage of participants that made any preparatory actions and the mean number of preparatory actions prior to a suicide attempt. If participants interpreted a “suicide plan” similarly, one would expect minimal overlap between distributions of planning characteristics for single-item “planners” and “non-planners.” Instead, each of the distributions overlap substantially (71–92%) using the bins in the current plot, suggesting participants do not reliably interpret the single-item suicide planning question.
Fig 4
Fig 4. Responses to single-item suicide attempt question and the results of coded narrative responses.
Single-item suicide attempt questions result in a 10.7% false positive rate and 6.39% false negate rate for the sample (9.91% false negative rate among ideators).

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Grant support

AJM received a Sackler Scholars Program in Psychobiology grant given to students within the Department of Psychology at Harvard University from the Dr Mortimer and Theresa Sackler Foundation. MKN received funding from the John D. and Catherine T. MacArthur Foundation. There is no specific grant number. The Sackler and MacArthur Foundations had no influence or input into any aspect of the study (e.g., planning or execution), writing of the manuscript or submission process.