midparental height discrepancy

[eatyourpeas]

For the following parental heights:
father: 167cm, mother 151cm, midparental centile should be between 9-25th centiles but is plotting lower

vs

[eatyourpeas]

Update from prof. wright:
The lookup (the nonagram on the paper charts) is based on the regression of both parental z scores on child’s height so to replicate this you have to convert each parents’ height to z scores, take the mean and multiply it by the regression coefficient which is 0.5 (or near as dammit) to give parental height prediction
So in summary, simplified, to (MatHtz +PatHtz)/4
Then subtract this from the child’s Z score if you want a measure in Z score units of how discrepant the child is.

[eatyourpeas]

Wright-1999-The strengths and limitations of p.pdf