# Exercises for Section 2.1 Discrete random variables#

This notebook contains the solutions to the exercises from Section 2.1 Discrete random variables of the No Bullshit Guide to Statistics.

## Notebooks setup#

import os
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
import seaborn as sns

# Pandas setup
pd.set_option("display.precision", 2)

# Figures setup
DESTDIR = "figures/prob"  # where to save figures

%config InlineBackend.figure_format = 'retina'


## Python functions defined in the text#

# define the probability mass function for the random variable D
def fD(d):
if d in {1,2,3,4,5,6}:
return 1/6
else:
return 0

import numpy as np

def fH(h):
lam = 20
return lam**h * np.exp(-lam) / np.math.factorial(h)

def FH(b):
return sum([fH(h) for h in range(0,b+1)])


## Exercies 1#

### E2.1#

sum([fD(d) for d in range(2,5+1)])

0.6666666666666666


### E2.2#

# define the probability mass function for the random variable D4
def fD4(d):
if d in {1,2,3,4}:
return 1/4
else:
return 0

[fD4(d) for d in range(1,4+1)]

[0.25, 0.25, 0.25, 0.25]


### E2.3#

ys = [1,2,3]
fYs = [0.2, 0.5, 0.3]

fig, ax = plt.subplots()
plt.stem(ys, fYs, basefmt=" ")
ax.set_xticks([1,2,3])
ax.set_xlim(0,4)
ax.set_yticks([0])
ax.set_xlabel('$y$')
ax.set_ylabel('$f_Y$')

Text(0, 0.5, '$f_Y$')

# fY(1), fY(2), fY(3) are
fYs

[0.2, 0.5, 0.3]


### E2.4#

Define $$A$$ as the set of outcomes $$\{5,6\}$$, which is a subset of the sample space $$\{1,2,3,4,5,6\}$$ for the random variable $$D$$. The complement of $$A$$ is $$A^c= \{1,2,3,4\}$$, and we know $$\Pr(A^c)= \frac{4}{6}$$. Applying the complement rule, we obtain $$\Pr(A) = 1 - \Pr(A^c) = 1 - \frac{4}{6} = \frac{2}{6}$$.

## Exercies 2#

### E2.5#

FH(30) - FH(10-1)

0.9815299064117703


### E2.6#

# define the probability mass function for the random variable D
def fD(d):
if d in {1,2,3,4,5,6}:
return 1/6
else:
return 0

def FD(b):
return sum([fD(d) for d in range(0,b+1)])

FD(1), FD(2), FD(3), FD(4), FD(5), FD(6)

(0.16666666666666666,
0.3333333333333333,
0.5,
0.6666666666666666,
0.8333333333333333,
0.9999999999999999)


### E2.7#

from scipy.stats import randint

rvD = randint(1, 6+1)
ds = np.linspace(0, 7, 400)
FDs = rvD.cdf(ds)

ax = sns.lineplot(x=ds, y=FDs)
ax.set_xlabel('b')
ax.set_ylabel('$F_D$')

Text(0, 0.5, '$F_D$')


### E2.8#

FH(18), FH(19), FH(20), FH(21), FH(22)

(0.3814219494471548,
0.47025726683924,
0.5590925842313252,
0.6436976484142636,
0.7206113431260257)


### E2.9#

FH(16), FH(17), FH(18)

(0.22107420155444096, 0.29702839792467384, 0.3814219494471548)


So $$F_H^{-1}(0.25) = 17$$. This is the smallest value $$b$$ for which $$F_H(b)$$ is greater than $$0.25$$.

# ALT. by calling the inverse-CDF function on rvH object
from scipy.stats import poisson
rvH = poisson(20)
rvH.ppf(0.25)

17.0


## Exercies 3#

### E2.10#

# define the probability mass function for the random variable C
def fC(c):
return 1/2
else:
return 0

def w(c):
return 1.90
else:
return 0

sum([w(c) * fC(c) for c in {"heads", "tails"}])

0.95


### E2.11#

def u(d):
if d == 6:
return 4
elif d == 5:
return 3
else:
return 0

sum([u(d)*rvD.pmf(d) for d in range(1,6+1)])

1.1666666666666665


### E2.12#

See derivation in the book.

### E2.13#

def fD20(d):
if d in range(1,20+1):
return 1/20
else:
return 0

# Expected value of the random variable D20
sum([d*fD20(d) for d in range(1,20+1)])

10.5


## Exercies 4#

### E2.14#

from scipy.stats import randint
rvD = randint(1,6+1)

[rvD.pmf(d) for d in range(1,6+1)]

[0.16666666666666666,
0.16666666666666666,
0.16666666666666666,
0.16666666666666666,
0.16666666666666666,
0.16666666666666666]

sum([rvD.pmf(d) for d in range(1,6+1)])

0.9999999999999999

# F_D(4) =
sum([rvD.pmf(d) for d in range(1,4+1)])

0.6666666666666666

# F_D(4) =
rvD.cdf(4)

0.6666666666666666

# mu =      sigma^2 =
rvD.mean(), rvD.var()

(3.5, 2.9166666666666665)

def w(d):
if d == 6:
return 5
else:
return 0

sum([w(d)*rvD.pmf(d) for d in range(1,6+1)])

0.8333333333333333

# Calling:
# rvD.expect(w)

# Raises an error:
# ValueError: The truth value of an array is ambiguous.
#             Use a.any() or a.all()


This error occurs because the Python function w chokes if we pass it an array of inputs to evaluate, which is what the method rvD.expect does (for efficiency reasons).

We can easily solve this by “vectorizing” the function using the NumPy helper method vectorize, which makes any Python function work with vector inputs (arrays of numbers).

# create a "vectorized" version of the function w
vw = np.vectorize(w)
rvD.expect(vw)

0.8333333333333333


### E2.15#

from scipy.stats import randint
rvD20 = randint(1, 20+1)

rvD20.pmf(7)

0.05

sum([rvD20.pmf(d) for d in range(1,4+1)])

0.2

rvD20.cdf(4)

0.2

rvD20.mean(), rvD20.var(), rvD20.std()

(10.5, 33.25, 5.766281297335398)


### E2.16#

from scipy.stats import poisson
rvM = poisson(40)

rvM.mean(), rvM.var(), rvM.std()

(40.0, 40.0, 6.324555320336759)

sum([rvM.pmf(m) for m in range(33,44+1)])

0.6503813622782281

rvM.cdf(44) - rvM.cdf(33-1)

0.6503813622782224

rvM.ppf(0.95)

51.0

rvM.rvs(10)

array([40, 34, 34, 50, 43, 36, 35, 53, 42, 50])