Need help explaining apparent conflicting results - We ran 2 different, but similar intervention type trials, each with their own control groups. Results were as follows (lower is better):
Trial 1: Treated n1=936,539 Treated conversion x1=52,223---->5.58% Control c1=124,599 Control conversion cx1=6,988---->5.61% Lift= -0.6%(insignificant) Trial 2: Treated n2=463,308 Treated conversion x2=116,816--->25.21% Control c2=52,218 Control conversion cx2=13,617---->26.08% Lift= -3.3%
Now if we were to combine the two trial to determine is there was a combined intervention effect we get:
Combined: Treated n=1,399,837 Treated conversion x=169,039--->12.08% Control c=176,817 Control conversion cx=20,605---->11.65% Lift=3.3%
So the combined effect is not showing a positive impact of the combined interventions, although the the two trials are either showing a positive effect or neutral.
Any ideas to describe this phenomenon?