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Table 3 Threats to internal validity (source: adapted from [12])

From: Evaluation of alternative school feeding models on nutrition, education, agriculture and other social outcomes in Ghana: rationale, randomised design and baseline data

Indicator Metric Spill-over and contamination Hawthorne and placebo effects
Schooling Enrolment, attendance, drop-out and completion Children may attend school from neighbouring communities to have access to meals Expectation of coming programme in control communities
Cognitive ability Raven’s matrices, digit span and/or other tests Very unlikely Teachers’ and children’s attempt to over-perform in both project and control communities
Attention Digit span and/or other test Very unlikely Teachers’ and children’s attempt to over-perform in both project and control communities
Learning achievement Scores on language and maths tests Very unlikely Teachers’ and children’s attempt to over-perform in both project and control communities
Physical growth Anthropometric measures of height and weight Children from other communities may access school meals Very unlikely
Physical activity levels (PAL) Parents’ perceptions Very unlikely Very unlikely
Diet diversity Household consumption Very unlikely Very unlikely
Micronutrient intake Iron status, anaemia Children from other communities may access school meals Very unlikely
Income Farm profits Unlikely, if food purchases are made in control communities Very unlikely