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The following is a table of frequency of occurrence of the alphabet in plain English text taken from a cryptanalysis text, and the length of Morse Code encoding of those characters.
Letter | Frequency (%) | Elements | Contribution |
E | 0.1300 | 4 | 0.520 |
T | 0.0920 | 6 | 0.552 |
N | 0.0790 | 8 | 0.632 |
R | 0.0760 | 10 | 0.760 |
O | 0.0750 | 14 | 1.050 |
A | 0.0740 | 8 | 0.592 |
I | 0.0740 | 6 | 0.444 |
S | 0.0610 | 8 | 0.488 |
D | 0.0420 | 10 | 0.420 |
L | 0.0360 | 12 | 0.432 |
H | 0.0340 | 10 | 0.340 |
C | 0.0310 | 14 | 0.434 |
F | 0.0280 | 12 | 0.336 |
P | 0.0270 | 14 | 0.378 |
U | 0.0260 | 10 | 0.260 |
M | 0.0250 | 10 | 0.250 |
Y | 0.0190 | 16 | 0.304 |
G | 0.0160 | 12 | 0.192 |
W | 0.0160 | 12 | 0.192 |
V | 0.0150 | 12 | 0.180 |
B | 0.0100 | 12 | 0.120 |
X | 0.0050 | 14 | 0.070 |
Q | 0.0030 | 16 | 0.048 |
K | 0.0030 | 12 | 0.036 |
J | 0.0020 | 16 | 0.032 |
Z | 0.0010 | 14 | 0.014 |
Total | 1.00000 | 9.076 |
Average word duration will be 5 * 9.076 + 7 or 52.38
This represents 60/52.38/10 s (114ms rounded to the ms) per dit interval (Baud) at 10WPM. With that figure, the WPM speed is 2.28 times the dit frequency
The PARIS test has 53 dit elements per word, so dit time of 113ms at 10WPM.
I note that an older ARRL handbook suggests for electronic keyers that the WPM speed is 2.4 times the dit frequency (120ms/dit at 10WPM), but it does not provide the derivation.