Think of the CD's strength as +2% and weakness -2% on top of the overall research percentage rating, you will always gain more points in the area of strength and less in the area of the weakness, other areas are standard rate.
4.5 level staff usually give 1% less than 5 star staff gives. But ultimately the closer you get to level 20 the lower of an advantage you will get in research.
For example
level 12 with 4.5 research in one area is 61% but 5star is 63%
level 11 with 4.5 research in one area is 62% but 5 star is 64%
level 10 with 4.5 research in one area is 63% but 5 star is 65%
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level 20 with 4.5 research in one area is 38% but 5 star is 40%
Other thing is, if you had lets say CD 5star and TD 4.5 then research will still say 4.5 but it will only have 1% less than full 5stars for both and the same if it was the other way around TD 5star and CD 4.5
But the CD is the most important Staff Member you really want 5star here even if they have a high priority area as a weakness in order to get the best starting base for the new season.
If you are feeling confused then this might help:
5 Star is best
4.5 star is 2nd best
4 star is 3rd best
3.5 is 4th best
3 star is 5th best
2.5 star is 6th best
2 star is 7th best
1.5 star is 8th best
1 star 9th best
0.5 star is 10th best
0.0 star is 11th best
Why didn't you just say that last bit in the first place you might be saying