Question

Here are the total scores of 10 students in an introductory statistics course.

62 93 54 76 73 98 64 55 80 71
Previous experience with this course suggests that these scores should come from a distribution that is approximately Normal with mean 72 and standard deviation 10. The grading policy says that the cutoffs for the other grades correspond to the following: bottom 5% receive F, next 15% receive D, next 35% receive C, and next 30% receive B. These cutoffs are based on the N(72, 10) distribution.
A) Give the cutoffs for the grades in this course in terms of standardized scores.
B) Give the cutoffs in terms of actual total scores.
C) Do you think that this method of assigning grades is a good one?

Answer 1

Kindly check explanation

Explanation:

Given that :

Mean (m) = 72

Standard deviation (sd) = 10

Grading cutoff policy:

bottom 5% receive F

next 15% receive D

next 35% receive C

next 30% receive B

A) Give the cutoffs for the grades in this course in terms of standardized scores.

Standardized score for grade cutoff:

Locating the Zscore for the proportions on the z table :

Bottom 5% = 0.05 ; corresponding Zscore = - 1.645

next 15% receive D = (5 +15)% 0.20 ; corresponding Zscore = - 0.84

next 35% receive C = (20+35)% = 0.55 ; corresponding Zscore = 0.13

next 30% receive B = (55 + 30)% = 0.85 ; Corresponding Zscore = 1.04

B) Give the cutoffs in terms of actual total scores.

Recall:

Zscore = (x - m) /sd ; where x = actual score

x = sd*z + m

For F:

10*(-1.645) + 72 = 55.55

For D:

10*(-0.84) + 72 = 63.6

For C:

10*(0.13) + 72 = 73.3

For B:

10*(1.04) + 72 = 82.4

C) Do you think that this method of assigning grades is a good one?

Yes, it is good in terms of expressing scores around the mean such that score below are negative and those above are positive. However, it is a little bit ambiguous.