Table 3.

List of para-data indices.

Index			Description
Survey completion
	Time lag from invitation to survey uptake Time of day survey was taken Missed survey Time from uptake to complete the survey	This category of indices measures aspects of survey completion, such as whether a survey is incomplete and the time spent on completing a survey. Emerging evidence suggests that these indices may be predictive of cognitive impairment [31].
Response time
	Median browser-side response time Deviations from the median browser-side response time Variability in response times across questions Autocorrelation (inertia) of response times across questions Item response theory person-fit statistics response time patterns Pause rate (no response for >20 seconds) Median pause duration	This category of indices measures aspects of response times such as the median response times, variability, and autocorrelations. A growing number of studies suggest that response times and their derived measures may be associated with cognitive impairment [32,33].
Errors and corrective behaviors
	Proportion of corrected or changed answers Rate of error messages received (eg, invalid entry) Rate of “back” button use Rate of “next” button use without a completed response	This category of indices measures aspects of errors and corrective behaviors in answering web-based surveys.
Mouse and touch efficiency
	Number of mouse clicks per page (median) [34] Variability of mouse clicks across screens [34] Total pixel count (inefficiency in movement: “wandering around on screen”) [34]	This category of indices measures aspects of mouse clicks and inefficiencies in mouse movement behaviors.
Keystrokes
	Median time between keystrokes (for text entries) [34] Variability of time between keystrokes (within a text entry) [34]	This category of indices measures temporal rhythms of keystrokes from keyboard entries by respondents for survey questions.