Help coming with a descriptive statistic

Question

I have some data about employee performance that I want to create a descriptive statistic for that can easily show at a glance how efficient the employee is.

Let's start with some sample data: (The process times are in seconds)

+----------+------------------+--------------+--------------+--------------+
| Employee | Avg Process Time |  Store Avg   | Buys Per Day | Total # Buys |
+----------+------------------+--------------+--------------+--------------+
| 1        | 291              | 376.8        | 4.4          | 22           |
+----------+------------------+--------------+--------------+--------------+
| 2        | 874              | 376.8        | 4            | 4            |
+----------+------------------+--------------+--------------+--------------+
| 3        | 291              | 376.8        | 2.5          | 10           |
+----------+------------------+--------------+--------------+--------------+
| 4        | 294              | 376.8        | 8            | 160          |
+----------+------------------+--------------+--------------+--------------+
| 5        | 1331             | 376.8        | 1            | 1            |
+----------+------------------+--------------+--------------+--------------+
| 6        | 284              | 376.8        | 3            | 42           |
+----------+------------------+--------------+--------------+--------------+
| 7        | 537              | 376.8        | 4.8          | 19           |
+----------+------------------+--------------+--------------+--------------+
| 8        | 354              | 376.8        | 8.7          | 78           |
+----------+------------------+--------------+--------------+--------------+
| 9        | 453              | 376.8        | 6.2          | 131          |
+----------+------------------+--------------+--------------+--------------+
| 10       | 299              | 376.8        | 2.6          | 26           |
+----------+------------------+--------------+--------------+--------------+
| 11       | 438              | 376.8        | 7.2          | 123          |
+----------+------------------+--------------+--------------+--------------+

So I want to come up with one descriptive statistic that shows how effective this employee is relative to the store average. Obviously lower times are better. I also want to factor in the amount of buys that they worked, and compensate for outliers like employee #5. I'd really like to get to a number that lies from 0-100 with 100 being the most efficient employee.

Can someone point me in the right direction for my research? I'm not looking for someone to write a formula for me necessarily. I just don't necessarily know how I should approach it.

Thanks!

EDIT:

Okay here is the way I am displaying the data right now. Those little bullet graphs on the right are useful for showing how each employee performs against the store average. What I want to add is a single number that takes the 'idea' of those graphs and takes into account the number of 'buys' (transactions) as well.

(PT is process time, PT/C is process time per container(bin). Each 'buy' or transaction can have one or more 'bins' and more 'bins' usually means more process time)

Answer 1

You could find the relative ranking of factors using z-scores or use the IQR in order to compare those in each observation, then create a composite score.

Here is an example considering average time and total buys with equal weight. I have calculated the z-score of each factor, found where they fall in the distribution (here, I am assuming normal), and created a composite (i.e. $(p_t + p_b)/ 2$). The interpretation of the composite is that an average worker will be at .5, a worker above or below that number is respectively more or less "efficient".

+-----+-----+-----+-----+-----+-----+-----+
|time |total|time-|buy-z|pt   |pb   |score|
|     |buys |z    |     |     |     |     |
+-----+-----+-----+-----+-----+-----+-----+
|291  |22   |-0.65|-0.62|0.74 |0.27 |0.51 |
+-----+-----+-----+-----+-----+-----+-----+
|874  |4    |1.21 |-0.95|0.11 |0.17 |0.14 |
+-----+-----+-----+-----+-----+-----+-----+
|291  |10   |-0.65|-0.84|0.74 |0.20 |0.47 |
+-----+-----+-----+-----+-----+-----+-----+
|294  |160  |-0.64| 1.90|0.74 |0.97 |0.86 |
+-----+-----+-----+-----+-----+-----+-----+
|1331 |1    |2.67 |-1.01|0.00 |0.16 |0.08 |
+-----+-----+-----+-----+-----+-----+-----+
|284  |42   |-0.68|-0.26|0.75 |0.40 |0.57 |
+-----+-----+-----+-----+-----+-----+-----+
|537  |19   |0.13 |-0.68|0.45 |0.25 |0.35 |
+-----+-----+-----+-----+-----+-----+-----+
|354  |78   |-0.45|0.40 |0.67 |0.66 |0.67 |
+-----+-----+-----+-----+-----+-----+-----+
|453  |131  |-0.13|1.37 |0.55 |0.91 |0.73 |
+-----+-----+-----+-----+-----+-----+-----+
|299  |26   |-0.63|-0.55|0.73 |0.29 |0.51 |
+-----+-----+-----+-----+-----+-----+-----+
|438  |123  |-0.18|1.23 |0.57 |0.89 |0.73 |
+-----+-----+-----+-----+-----+-----+-----+