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CIE A Level Maths: Probability & Statistics 1復(fù)習(xí)筆記3.2.1 The Binomial Distribution

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年8月18日下午12:25

Properties of Binomial Distribution

What is a binomial distribution?

A binomial distribution is a?discrete probability distribution
The?discrete random variable?X?follows a?binomial distribution?if it?counts the number of successes?when an experiment satisfies the conditions:

What are the important properties of a binomial distribution?

- Square root to get the standard deviation
The distribution can be represented visually using a vertical line graph
- If?p?is?close to 0?then the graph has a?tail to the right
- If?p?is?close to 1?then the graph has a?tail to the left
- If?p?is?close to 0.5?then the graph is?roughly symmetrical
- If?p =?0.5?then the graph is?symmetrical

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4-2-1-the-binomial-distribution-diagram-1-part-3-1

Modelling with Binomial Distribution

How do I set up a binomial model?

Identify?what a?trial?is in the scenario
- For example: rolling a dice, flipping a coin, checking hair colour
Identify?what the?successful outcome?is in the scenario
- For example: rolling a 6, landing on tails, having black hair
Make sure you?clearly state?what your?random variable?is
- For example, let?X??be the number of students in a class of 30 with black hair

What can be modelled using a binomial distribution?

Anything that satisfies the?four conditions
Sometimes it might seem like there are more than two outcomes
- For example, let?Y? be the number of yellow cars that are in a car park full of 100 cars
- Although there are more than two possible colours of cars, here the trial is whether a car is yellow so there are two outcomes (yellow or not yellow)
- Y?would still need to fulfil the other conditions in order to follow a binomial distribution
Sometimes a sample may be taken from a population
- For example, 30% of people in a city have blue eyes, a sample of 30 people from the city is taken and?X??is the number of them with blue eyes
- As long as the population is large and the sample is random then it can be assumed that each person has a 30% chance of having blue eyes

What can not be modelled using a binomial distribution?

Anything where the number of trials is?not fixed?or is?infinite
- The number of emails received in an hour
- The number of times a coin is flipped until it lands on heads
Anything where the outcome of?one trial affects?the outcome of the?other trials
- The number of caramels that a person eats when they eat 5 sweets from a bag containing 6 caramels and 4 marshmallows
  - If you eat a caramel for your first sweet then there are less caramels left in the bag when you choose your second sweet
Anything where there are?more than two outcomes?of a trial
- A person's shoe size
- The number a dice lands on when rolled
Anything where the?probability of success changes
- The number of times that a person can swim a length of a swimming pool in under a minute when swimming 50 lengths
  - The probability of swimming a lap in under a minute will decrease as the person gets tired

Worked Example

It is known that 8% of a large population are immune to a particular virus. Mark takes a sample of 50 people from this population. Mark uses a binomial model for the number of people in his sample that are immune to the virus

(a)State the distribution that Mark uses.

(b)State the two assumptions that Mark must make in order to use a binomial model.

4-2-1-the-binomial-distribution-we-solution-part-1

4-2-1-the-binomial-distribution-we-solution-part-2

Exam Tip

If you are asked to criticise a binomial model always consider whether the trials are independent, this is usually the one that stops a variable from following a binomial distribution!

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