The sealed test envelope

Can a rule pass the cards it practised and fail new cards?

Make your own investigation

Seal four new cards before selecting a rule. Record the rule before opening the envelope.

Select a size rule on practice cards, then test it on sealed cards without changing the rule.

Case cards

Cut these cards apart. Use the original example first, then let a partner supply a changed case. Keep sealed/final cards with the teacher until the learner commits a rule.

What this cannot establish

Once sealed results influence your choice, those cards become development data.