## 4 Materials and Methods
### Data Analysis  
The search task was considered to be completed correctly if the target's corresponding quadrant was chosen. Thus, for all trials, the data of true-false responses (in the form of a 1 or 0) were recorded. The participants' visual search outcome was the number of correct responses and had a range of 0 (i.e., no correctly completed tasks) to 5 (i.e., all tasks correctly completed), with intervals of 1 for each combination of the physical variables. The search time was calculated as the time starting from the initial demonstration of search items on the volumetric display until the space button was pressed.  
To investigate the search strategy and the image manipulation requirements of participants, the search pattern was recorded in the form of the image slice sequence that was shown on the closest plane of the volumetric display. Afterwards, we analyzed the following three parameters to describe image manipulation and search pattern: The number of clicks, direction changes, and image coverage. The number of clicks represents the frequency of image manipulations during the search procedure. The direction changes were defined as each time the observer changed the direction of image manipulation. For instance, if the participant pressed the up arrow key several times and then pressed the down arrow key, one direction change was counted.  
The manipulations could cover different amounts of image slices. We were interested in assessing to what extent images were manipulated and how this approach was influenced by the stimulus physical properties for participants with different amounts of experience in radiology. We defined the types of image coverage as following: \"No changes\" meant that no clicks were employed and the participant viewed the entire three-dimensional image on the display; \"half image\"--1 to 5 slices were moved within the image manipulations; and \"full image\"--more than 5 slices were moved.  
The data analysis was performed using R Statistical Software version 3.5.2. (Foundation for Statistical Computing, Vienna, Austria). The differences in the number of correct responses and direction changes were analyzed separately by pairwise comparisons for each of the factors, using non-parametric rank-based tests, namely, the Wilcoxon signed rank test for the repeated measures factors (target-distractor similarity and set size), and the Wilcoxon rank sum test for the between-groups factor (experience-adjacent group). To control the family-wise error rate, pairwise comparison \(p\)-values were adjusted via the Bonferroni procedure.  
We evaluated search time and the number of clicks using a three-way mixed design analysis of variance (ANOVA) based on two repeated-measure factors (target-distractor similarity and set size), and between-group factor (experience-adjacent group). Given the positive skew of the distributions, the search time and number of manipulations were log-transformed prior to the ANOVA analysis. For the post hoc comparisons, pairwise t-tests with Bonferroni correction were used.  
To analyze the association between image coverage, experience-adjacent group, and stimulus physics, we performed Cochran-Mantel-Haenszel tests of two associations between three categorical variables, namely, (1) image coverage, experience-adjacent group, and set size, and (2) image coverage, experience-adjacent group, and target-distractor similarity. For the post hoc comparisons, the associations between the two variables of interest (image coverage type and set size; image coverage type and target-distractor similarity) were analyzed separately with a chi-squared test of association for each of the participants' groups. The multiple comparison \(p\)-values were adjusted via the Bonferroni procedure.