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Lack of blinding causes the dilutional effect and observer's (ascertainment bias in the example given in the endgame (Cluster randomized trial where the classes were taken as clusters in 6 selected schools). My contention is that if clusters are taken as classes within a school with two interventions in the same school then contamination (I think it is the same as dilutional effect in the context) is bound to occur. This could have been avoided if one school was considered as one cluster, i.e. all classes in one school had one intervention (treatment or placebo). If two interventions are being carried out simultaneously in closeby clusters then contamination (or may be dilution, in other words) is bound to occur as blinding may not be possible sometimes as in behavioral change in obesity programmes if the clusters closeby.