Statistics 1 Dersi 5. Ünite Sorularla Öğrenelim

Define probability distribution.

The probability distribution of a discrete random variable is a list of odds associated with each of its possible values. It is also called the probability function or the probability mass function. Basically we can define probability function as a rule that assigns probabilities to the values of random variables. For a discrete random variable X, probability function (probability mass function) must satisfy the following properties,

1. The probability of each value of the discrete random variable X must be between or equal to 0 and 1. 0 ? P (X = x) ? 1 for all
2. The sum of the probabilities of each outcome of the random variable must equal to 1.

Is the following function satisfy the conditions to be a probability distribution?

No. For all X it satisfies 0 ? P (X = x) ? 1 but the sum of P(X=x)=0,2+0,1+0,4+0,4=1,1>1. So it is not a probability distribution function.

Define cumulative distribution function.

Cumulative distribution function (CDF) of a discrete random variable X, denoted by F (x) and defined as,F(x)=P(X?x)=xi?x?P(X=xi)This expression for cumulative distribution function implies that it’s a cumulative probability value for random variables which are less than or equal to specific random value of xi . Through the definition of CDF, it also supplies probability values through utilizing probability mass function.For a discrete random variable X, CDFF (X) the following properties are shown.

1. 0 ?F (x) ? 1 and
2. If x1 ? x2 then F (x1) ? F (x2)

Find the cumulative distribution function for the following probability distribution.

Assume that P(X=x)=x/10 for X=,1,2,3,4. Find P(2?X<4)

P(2?X<4)=P(X=2)+P(X=3)=2/10+3/10=0.5

Calculate the mean and variance for random discrete variable X which has the following distribution:

Define the binomial distribution.

Binomial distribution is a widely employed discrete probability distribution in statistics where a set of independent observations constitutes exactly two disjoint outcomes of a trial. Therefore in binomial distribution, an outcome of a random experiment can be classified under two different categories. For example, when a die is tossed once we observe six different numbers, that is x = 1, 2, 3, 4, 5 and 6. If we classified these outcomes as even or odd numbers then we will get two different outcomes.

What is a Bernoulli trial, which properties does it have?

A random experiment (trial) with only two possible outcomes is frequently used and called a Bernoulli trial with the following properties:

1. Each trial has only two possible outcomes, such as head and tail, 0 and 1 or success and failure.
2. The trials are independent experiments.
3. The probability of success, denoted by and the probability of failure, denoted by remains constant for all trials (p + q = 1).

What is the mean and variance of a binomial distribution?

The mean (µ) and variance (?2) of the binomial random variable X ~Binomal (n, p) can be calculated from these formulas

µ = E (x) = np and ?² =V (X) = np (1 - p)

A coin is flipped 4 times. What is the probability of getting at least 3 heads?

At least 3 heads means 3 heads or 4 heads. Thus we have to calculate:

P(H=3)+P(H=4)

P(H=4)=C(4,4)0.5⁴0.5⁰=1/16

P(H=3)=C(4,3)0.5³0.5¹=4/16

P(H=3)+P(H=4)=1/16+4/16=5/16

The probability of a student passing statistics course is 60 %. If 4 students take the statistics course what is the probability of at most 2 students being succesfull?

P(X ? 2)=P(X=0)+P(X=1)+P(X=2)

P(X=0)=C(4,0)*0.6⁰*0.4⁴=0.0256

P(X=1)=C(4,1)*0.6¹*0.4³=0.1536

P(X=2)=C(4,2)*0.6²*0.4²=0.3456

so:

P(X ? 2)=P(X=0)+P(X=1)+P(X=2)=0.0256+0.1536+0.3456=0.5248

The probability rain in spring in any day is 80 %. What is the probability of 2 rainy days out of 3 days randomly choosen in spring season?

P(X=2)=C(3,2)*0.8²*0.2¹=3*0.64*0.2=0.384=38.4%

In which cases do we use Poisson distribution?

The Poisson distribution is widely used for discrete probability distribution which is used to model the number of outcomes occurring during a specified time interval or in a definite region. Here, the time interval indicates any length, for instance an hour, a day, or a month. Also, the definite region term means that a piece of line segment, an acre, size of a football field, a volume.

What are the mean and variance of the Poisson distribution?

The mean (µ) and variance (?²) of a Poisson random variable X ~ Poisson (?) with parameter ? can be calculated using the following formulas,

µ = E (x) = ?

?² = V (X) = ?

An autopark receives 5 automobiles per hour in average. What is the probability of at most 2 automobiles entering that autopark in an hour?

P(X ? 2)=P(X=0)+P(X=1)+P(X=2)

for ?=5

P(X=0)=(e^-5*5⁰)/0!=0.0067

P(X=1)=(e^-5*5¹)/1!=0.0337

P(X=2)=(e^-5*5²)/2!=0.0842

So:

P(X ? 2)=P(X=0)+P(X=1)+P(X=2)=0.124

What is the difference between hypergeometric distribution and binomial distribution? In which area is hypergeometric disttribution used?

In binomial distribution the sampling process is carried out with replacement. On the other hand hypergeometric distribution doesn’t involve with independence assumption for each trial and accordingly the sampling process is established on without replacement. Because of these features, hypergeometric distribution is broadly used in various real life applications especially for acceptance sampling in quality control