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A neuroscientist measures the reaction times (in seconds) during an experimental session in a sample of cocaine-addicted (n = 8), morphine-addicted (n = 6), and heroin-addicted rats (n = 12). Mean reaction times in each sample were 12, 13, and 17 seconds, respectively. What is the weighted mean for all three samples? Hint: The overall mean is not 14.0 seconds. (Round your answer to two decimal places.)

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rodolforodriguezr

Answer:

14.54

Step-by-step explanation:

In a weighted average a “weight” is associated to each value.

The weighted mean is computed the following way:

Suppose you want to calculate the weighted mean of the values  

a1, a1, …, an

with weights

w1, w2,..., wn

Then  

Weighted mean =(w1*a1+w2*a2+... +wn*an)/(w1+w2+...+wn)

In this case we have  

a1= 12, w1 = 8

a2 = 13, w2 = 6

a3 = 17, w3 = 12

Weighted mean  

(12*8+13*6+17*12)/(8+6+12) = 14.54 rounded to 2 decimal places.

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