Description
This web activity consisted of 72 test items, designed to measure aspects of memorisation, recognition and perception of images of cars.
Each group of items began with a stimulus, consisting of images of the car(s) to be memorised and recognised, presented on screen for a fixed number of seconds. Each subsequent test item comprised a set of three car images, numbered 1 to 3; one of the three images was a correct match with a car in the initial stimulus, while the other two images were incorrect matches. The twin was required to select the image that s/he thought was the correct match, using the keyboard to press key 1, 2 or 3. On pressing one of these keys, the web test automatically moved on to the next item or the next stimulus.
For reasons of copyright, the full item descriptions are not reproduced here. For details of the test items, please refer to the original published measures (see 18 Year measures page for details).
The test was divided into a practice and three main sections, which were presented consecutively in this order:
| Order | Section | No. of items | Stimulus | Description of items |
|---|---|---|---|---|
| 1 | Practice | 3 | 3 images of a single car; each image was shown for 3 seconds | Following the 3 stimulus images, there were 3 items testing recognition of the car. |
| 2 | Memorisation | 18 | 3 images each for 6 different cars; each image was shown for 3 seconds | Following the 3 stimulus images for each car, there were 3 items testing recognition of the car. |
| 3 | Clean | 30 | A single image showing 6 cars (the same 6 cars as in section 1). The image was shown for 20 seconds. | Following the stimulus image, there were 30 consecutive items, each testing recognition of one of the 6 cars, using clean images. |
| 4 | Degraded | 24 | The same image of 6 cars as used in section 2. The image was shown for 20 seconds. | Following the stimulus image, there were 24 consecutive items, each testing recognition of one of the 6 cars, using degraded images. |
The responses and scores for the 3 practice items were not recorded in the data, and do not form part of the total test score. After the practice items, twins were given feedback to tell them whether they gave correct responses (no such detailed feedback was given in the main test). Twins were required to answer all 3 practice items correctly before proceeding to the main test. If one or more practice items were answered incorrectly, then twins were required to repeat the 3 practice items.
Web Test Rules
This simple web test had no special rules. The 72 items and their associated stimuli were presented on screen, one at a time, in a fixed order that was the same for all twins. All 72 items were compulsory: if an item was not answered, it was not possible to proceed to the next item. There was no branching, no discontinue rule, and no time-out rule.
The test was designed to be completed in one session. All responses were recorded in the browser (using javascript) and only submitted to the web server at the end of the test. Hence, if the questionnaire was halted part-way through, for example by closing the browser, then all responses were lost; in this case, a twin would have to re-start the test from scratch on logging in again. If a twin attempted to repeat a question, using the browser's Back or Refresh buttons, then a warning message would appear in a pop-up box; if the twin ignored the message, then the test would be halted, and the twin would have to re-start the entire test from scratch.
Dataset Item Variables
The item variables relating to the Cars Perception test, as listed in the table below, were generated automatically by programs on the web server, with two exceptions. The exceptions (rcpcrtime1/2, rcpcrtmd1/2) were derived during dataset construction, and are included here to complete the set of variables relating directly to this activity. Note that the dataset only includes data for completed tests. If a twin started the test but left it unfinished, then the data were not submitted to the web server and were therefore not recorded. The start and end dates/times have not been retained in the dataset.
| Variables | Explanation | Values |
|---|---|---|
| rcpcrdata1/2 | Data flag: was the test completed? | 0=no, 1=yes |
| rcpcrsttm1/2 | Start date and time of the test [not in dataset] | Date-time values |
| rcpcrentm1/2 | End date and time of the test [not in dataset] | Date-time values |
| rcpcrtime1/2 | Time interval from start to end of the test [DERIVED VARIABLE] | Minutes (decimal values) |
| rcpcrtmd1/2 | Median item response time [DERIVED VARIABLE] | Seconds (decimal values) |
| rcpcrWXYZr1/2 | Test item response for item WXYZ* | 1, 2, 3 (denoting the image selected) |
| rcpcrWXYZs1/2 | Test item score for item WXYZ* | 0=incorrect, 1=correct |
| rcpcrWXYZt1/2 | Test item response time for item WXYZ* | Milliseconds (integer values) |
| rcpcrmemt1/2 | Memorisation score: total score for the first 18 items | 0 to 18 (integer values) |
| rcpcrclnt1/2 | Clean score: total score for items 19 to 48 | 0 to 30 (integer values) |
| rcpcrdegt1/2 | Degraded score: total score for items 49 to 72 | 0 to 24 (integer values) |
| rcpcrt1/2 | Test total score: total score for all 72 items | 0 to 72 (integer values) |
* WXYZ denotes the item code (a string of 3 or 4 letters and digits).
In the Memorisation section, each of six stimuli were followed by 3 questions (items). In each item code, the stimulus is represented by the letter t and is then numbered 1 to 6; the question (item) is then represented by the letter v and is then numbered 1 to 3. Hence, the Memorisation item codes are t1v1, t1v2, t1v3, t2v1, t2v2, t2v3, and so on.
In the Clean and Degraded section, the item codes have the letter t as prefix followed by numbers 01 through to 54, in the order that the items were presented on screen. The 30 Clean items have codes t01 to t30. These were followed by the 24 Degraded items with codes t31 to t54.