Learning rates and known-to-unknown flash-card ratios: comparing effectiveness while holding instructional time constant.

The team compared two flash-card drills. One deck had only new words. The other deck mixed new words with easy, known words. Each drill lasted exactly three minutes.

They used an alternating-treatments design. Kids got both types of drills in the same session. The order flipped each day.

Kids learned more words with the all-unknown deck. The mixed deck felt easier, but it taught fewer new words in the same three minutes.

Efficiency mattered. When time is tight, every second should target new material.

Born et al. (1974) also held time constant. College students in a PSI course learned slightly more than lecture students while using the same study hours. Both studies show that how you slice the minutes matters more than the clock.

Iwata (1988) and Clark et al. (1977) both found that extra verbal steps added no benefit. The flash-card study agrees: adding known words is like adding extra talk—busy work that steals time from new learning.

Bartle et al. (2026) trained staff with videos. Exemplars plus nonexemplars beat exemplars-only. The flash-card study flips that pattern: mixing easy items with new items slowed kids down. The difference is the learner. Staff videos aim for broad discrimination, so varied examples help. Kids drilling words need many quick reps of the exact target.

If you have only a few minutes, skip the comfort items. Run pure unknown flash-cards and you will see faster word growth. Save the known words for praise or review games after the drill.