If you are referring to one property but are giving the respective values for two different things do you use the singular or plural form? Can the property be treated as a mass noun so that it takes on a cumulative reference?
The yields of 1 and 2 were 76 and 89%, respectively.
The yield of 1 and 2 was 76 and 89%, respectively.
In control and treated cells, the mean values were 8.6 and 7.5, respectively.
In control and treated cells, the mean value was 8.6 and 7.5, respectively.
It's clear that the plural is used for cases in which two properties are listed (i.e., the yield and recovery rate of A were...) but it's not clear when only one propery is listed.