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short communications
In the paper by Sadler [J. Appl. Cryst. (1983). 16, 519–523] a factor of 2 was omitted in error in equation (10), which should read: 〈z2〉 = 〈a〉 + 〈b〉[1 + 2/〈P2(γ)〉]/3. The results in Table 1 were derived correctly from the data.