This Campbell methods policy note on synthesizing partial effect sizes sets out current Campbell Collaboration policy on the synthesis of partial effect sizes (i.e. adjusted effect sizes) in Campbell systematic reviews of intervention effects. It is intended as a point of reference for Campbell authors and editors in the design and production phases of Campbell reviews of intervention effects, to ensure that protocols and full reviews are prepared in line with policy. To provide context for the current policy, the note is prefaced with a brief introduction to the topic and appended with a list of additional resources. It is not intended to provide comprehensive guidance or a tutorial on how to apply methods in Campbell or other systematic reviews