Measures Wundt used to ensure 'controlled experimental conditions'
1. Thoroughly trained all ppts in the same way in the methods of introspection
2. All ppts went through the same procedure, in the same order
3. All ppts within each specific study were exposed to the same stimulus for the same amount of time
4. All ppts within each specific study received the same standardised instructions, to ensure their experience was not qualitatively different