Similarly we may consider only the vertical and lateral errors, thus obtaining a vertical in lieu of a horizontal rectangle containing 25 per cent of all the shots.
The 50 per cent "breadth column" should, in practice, generally be neglected, as most of the errors in shooting are always over and under, and not lateral, ones.
The Point of Mean Impact is the intersection of the lines of mean range and mean lateral deviation.
The Probable Rectangle is one which contains 50 per cent of all the shots.
To find the probable rectangle for any gun, multiply the mean error in range by 2.637 for the side of the rectangle parallel to the range, and the mean error in lateral deviation by 2.637 for the side perpendicular to the range.
In all range tables for guns the 50-per-cent zones for length, breadth, and height should be given; and by means of them and the following table of probability factors the dimensions of zones of other percentage, and also the percentage due to certain dimensions at different ranges, can be obtained.
PROBABILITY FACTORS.
| Percentage. | Factor. | Percentage. | Factor. | Percentage. | Factor. |
| 1 | .02 | 25 | .47 | 60 | 1.25 |
| 3 | .06 | 30 | .57 | 70 | 1.54 |
| 5 | .09 | 35 | .67 | 80 | 1.9 |
| 10 | .18 | 40 | .78 | 90 | 2.44 |
| 15 | .28 | 45 | .89 | 95 | 2.91 |
| 20 | .38 | 50 | 1.00 | 100 | ∞ (say 4) |
First.—Opposite any given percentage, say 20, we find in the contiguous column a factor, .38. If we multiply the dimensions of the 50-per-cent zone, given in the range table, by this factor .38, we will obtain the corresponding dimension of the 20-per-cent zone.