Interleaved Dot and Triangle Under Time Pressure
On a timed EPSO-style set you have 40 seconds left for one item. The sequence has eight frames in a row. A black dot sits on one of eight equally spaced marks around the perimeter of a square, numbered clockwise 1–8 from the top-left corner. Across frames 1–8 the dot appears at 1, 2, 3, 6, 5, 2, 7, 6. In the centre, a grey triangle points up in frames 1, 3, 5, 7 and down in frames 2, 4, 6, 8. Four answer options show frame 9; all four place the dot at a perimeter mark and the triangle up or down. Your scratch notes read 'dot +1 clockwise; triangle alternates' but the dot rule already fails at frame 4. The item is the last in the section, so there is no review time. You have time to test one alternative before the timer ends.
1. Set out the competing dot rules and decide which rule is better supported by the whole display.
Read the worked answer
List the dot positions by frame parity:
- odd frames 1, 3, 5, 7: positions 1, 3, 5, 7 - a step of +2 clockwise;
- even frames 2, 4, 6, 8: positions 2, 6, 2, 6 - a step of +4 clockwise, a half turn.
The triangle orientation is also constant within parity: odd frames up, even frames down. A single adjacent-step rule for the dot fails: the successive differences are +1, +1, +3, −1, −3, +5, −1, so the note '+1 clockwise' is contradicted at frame 4 and elsewhere. The interleaved reading explains both the dot and the triangle with one structural idea and accounts for the apparent anomaly. For frame 9, use the odd series: 7 + 2 = 9, which wraps to position 1; the triangle remains up.
2. Option A places the dot at 1 with the triangle up; B at 3 with the triangle up; C at 6 with the triangle down; D at 1 with the triangle down. Which do you mark, and why?
Read the worked answer
Mark option A: dot at 1, triangle up.
Option B is the strongest distractor because 3 is what you get by applying the even-frame step +4 to the odd-frame position 7: 7 + 4 = 11, which wraps to 3. That mixes the two series. Frame 9 is odd, so it must use the +2 step, not the +4 step. Option D keeps the correct dot but assigns the wrong parity to the triangle; frame 9 is odd and odd frames are up. Option C repeats a wrong dot and also flips the triangle. The safe check is to identify parity first, then apply that parity's step; do not carry over the step from the immediately preceding frame.