Finding standard deviation using only mean, min, max?

Question

I want to find the standard deviation:

Minimum = 5
Mean = 24
Maximum = 84

Overall score = 90

I just want to find out my grade by using the standard deviation

Thanks,

Answer 1

A standard deviation cannot in general be computed from just the min, max, and mean. This can be demonstrated with two sets of scores that have the same min, and max, and mean but different standard deviations:

• 1 2 4 5 : min=1 max=5 mean=3 stdev≈1.5811
• 1 3 3 5 : min=1 max=5 mean=3 stdev≈0.7071

Also, what does an 'overall score' of 90 mean if the maximum is 84?

Answer 2

I actually did a quick-and-dirty calculation of the type M Rad mentions. It involves assuming that the distribution is Gaussian or "normal." This does not apply to your situation but might help others asking the same question. (You can tell your distribution is not normal because the distance from mean to max and mean to min is not close). Even if it were normal, you would need something you don't mention: the number of samples (number of tests taken in your case).

Those readers who DO have a normal population can use the table below to give a rough estimate by dividing the difference of your measured minimum and your calculated mean by the expected value for your sample size. On average, it will be off by the given number of standard deviations. (I have no idea whether it is biased - change the code below and calculate the error without the abs to get a guess.)

    Num Samples   Expected distance      Expected error
             10                1.55                0.25
             20                1.88                0.20
             30                2.05                0.18
             40                2.16                0.17
             50                2.26                0.15
             60                2.33                0.15
             70                2.38                0.14
             80                2.43                0.14
             90                2.47                0.13
            100                2.52                0.13

This experiment shows that the "rule of thumb" of dividing the range by 4 to get the standard deviation is in general incorrect -- even for normal populations. In my experiment it only holds for sample sizes between 20 and 40 (and then loosely). This rule may have been what the OP was thinking about.

You can modify the following python code to generate the table for different values (change max_sample_size) or more accuracy (change num_simulations) or get rid of the limitation to multiples of 10 (change the parameters to xrange in the for loop for idx)

#!/usr/bin/python
import random

# Return the distance of the minimum of samples from its mean
#
# Samples must have at least one entry
def min_dist_from_estd_mean(samples):
    total = 0
    sample_min = samples[0]
    for sample in samples:
        total += sample
        sample_min = min(sample, sample_min)
    estd_mean = total / len(samples)
    return estd_mean - sample_min # Pos bec min cannot be greater than mean


num_simulations = 4095
max_sample_size = 100

# Calculate expected distances
sum_of_dists=[0]*(max_sample_size+1) # +1 so can index by sample size
for iternum in xrange(num_simulations):
    samples=[random.normalvariate(0,1)]
    while len(samples) <= max_sample_size:
        sum_of_dists[len(samples)] += min_dist_from_estd_mean(samples)
        samples.append(random.normalvariate(0,1))
expected_dist = [total/num_simulations for total in sum_of_dists]

# Calculate average error using that distance
sum_of_errors=[0]*len(sum_of_dists)
for iternum in xrange(num_simulations):
    samples=[random.normalvariate(0,1)]
    while len(samples) <= max_sample_size:
        ave_dist = expected_dist[len(samples)]
        if ave_dist > 0:
            sum_of_errors[len(samples)] += \
                abs(1 - (min_dist_from_estd_mean(samples)/ave_dist))
        samples.append(random.normalvariate(0,1))
expected_error = [total/num_simulations for total in sum_of_errors]

cols="    {0:>15}{1:>20}{2:>20}"
print(cols.format("Num Samples","Expected distance","Expected error"))
cols="    {0:>15}{1:>20.2f}{2:>20.2f}"
for idx in xrange(10,len(expected_dist),10):
    print(cols.format(idx, expected_dist[idx], expected_error[idx]))