Table 2.

Number of participants and clusters required in a multilevel study

Estimated ICC	Calculated DEFF	Number of children	Number of schools
0.00 - Base model	1	260	15
0.05 - Two-level model	2.15	559	23(36)a

ICC the intraclass correlation coefficient, n number of pupils, DEFF design effect = 1 + (nc - l)*ICC, (nc average number of individuals in a school = 24); The first value (260) is from Altman [26], page 456, and the next figures are multiplied with the calculated DEFF value

^aThe first number denotes the required sample size given the power calculation. The parenthetical number denotes the actual number of schools recruited for the study based on other practical considerations