Mathematics for Computer Science

Chapter 17 Random Variables584

f20;:75.k/

0:25

0:2

0:15

0:1

0:05

0

k

0 5 10 15 20

Figure 17.5 The pdf for the general binomial distributionfn;p.k/fornD 20
andpD:75.

The General Binomial Distribution

If the coins are biased so that each coin is heads with probabilityp, then the number
of heads has ageneral binomial density functionspecified by the pdf

fn;pWf1;2;:::;ng!Œ0;1ç

where

fn;p.k/D

n

k

!

pk.1p/nk:

for somen 2 NCandp 2 Œ0;1ç. This is because there are

n

k

sequences with
kheads andnktails, but now the probability of each such sequence ispk.1
p/nk.
For example, the plot in Figure 17.5 shows the probability density function
fn;p.k/corresponding to flippingnD 20 independent coins that are heads with
probabilitypD0:75. The graph shows that we are most likely to getkD 15
heads, as you might expect. Once again, the probability falls off quickly for larger
and smaller values ofk.