There is a lot of debate whether questions should be presented on a grid or in a single item per screen. From just an operational point of view, grids take less time to complete, which should decrease response burden, although new research shows that respondents seem to prefer a single item per screen. From a measurement point of view, grids pose numerous issues: higher item non-response, higher Cronbach's alpha, higher item non-differentiation, sometimes higher measurement error but not always.

In this experiment we are testing the Vitality (4 items) and Mental Health (5 items) subscales of the SF-36v2®. The SF-36v2Health Survey asks 36 questions to measure functional health and well-being from the patient's point of view. It is called a generic health survey, because it can be used across age (18 and older), disease, and treatment group, as opposed to a disease-specific health survey which focuses on a particular condition or disease. Two of the four items of the vitality scale and two out of five items of the mental health scale are reversed in meaning.

A sample of 2,500 KnowledgePanel® respondents was randomly assigned to one of five experimental conditions: Group 1: Standard grid; Group 2: Shaded grid; Group 3: One item per screen with horizontal response options; Group 4: One item per screen with vertical response options; Group 5: One item per screen with vertical shaded response options. Approximately 360 respondents completed the survey per condition for a completion rate of 73.4%.

The survey was optimized to be seen on a screen with minimum resolution of 800 by 600 pixels. During the study we collected the browser type for each respondent. This allowed us to exclude cases in which the survey was taken either on a MSNTV or on an iPhone/pda. The final sample used for the analysis, after exclusions, was of 1,449 cases for an average group size of 290.

We hypothesized that items presented on a grid would lead to more measurement error indicated by a lower Cronbach's alpha for and higher "inconsistencies" for the grid presentation. The main reason is that when items are presented on a single screen they allow the respondent to focus more on each question. When items are on a grid it is easier to get confused especially when the meaning of some of them is reversed. The index of consistency was in fact computed by correlating the total sum of scores for the reversed item with the total sum of scores for the non reversed items. If respondents are consistent in their answers the correlation should be higher.

Results are going in the expected directions (lower alpha level for the grid presentation and higher correlation for the single item presentation), although the differences among groups do not reach statistical significance.