How is the average platelet count per field calculated in this procedure?

• A. Multiply by 10
• B. Add counts and divide by 10
• C. Add counts and divide by 5
• D. Subtract from 10

Answer: (B)

The key idea is to find the mean by distributing the total across the number of observations you made. In this platelet counting procedure, platelets are counted in several high-power fields—typically ten. To get the average per field, you add all the counts from those fields and divide by ten. This gives the mean number of platelets observed per field, which is the value used for further estimation.

Multiplying the total by ten would inflate the value, since you’re not scaling up but averaging. Dividing by five would assume you only counted five fields, which isn’t the case. Subtracting from ten has no meaningful interpretation in calculating an average.