Binomial Distribution
Learn how to calculate probabilities for repeated independent trials with two outcomes.
Definition
- = number of trials
- = number of successes
- = probability of success on each trial
- = number of ways to choose successes from trials
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Worked Examples
You flip a fair coin 5 times. What is the probability of getting exactly 3 heads?
Identify the parameters
(flips), (heads), (fair coin) → n=5, k=3, p=0.5
Calculate the combination
→ 10 ways
Calculate success probability
→ 0.125
Calculate failure probability
→ 0.25
Multiply all parts
→ 0.3125 or 31.25%
Answer: The probability of getting exactly 3 heads in 5 flips is or
Common Mistakes
Forgetting to include the combination
Why it's wrong: The combination counts HOW MANY WAYS to arrange successes among trials. Without it, you only calculate the probability of ONE specific arrangement.
Correct: Always use the full formula:
Using for both success AND failure terms
Why it's wrong: The failure probability is , not . If success is 0.3, failure must be 0.7.
Correct: Use for successes and for failures.
Confusing 'at least' with 'exactly'
Why it's wrong: 'Exactly 3' means , but 'at least 3' means
Correct: For 'at least', add up probabilities for all values from that number to . Or use complement:
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Interactive Sandbox
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Practice Problems
17 problemsWhich of the following is a condition for a binomial distribution?
Why It Matters
- Quality Control: What's the probability that 3 out of 100 products are defective?
- Medicine: If a drug works 80% of the time, what's the chance it helps at least 7 out of 10 patients?
- Sports: What's the probability a basketball player makes exactly 6 out of 10 free throws?
- Genetics: If a trait has 25% inheritance probability, how likely is it that 2 out of 4 children inherit it?
Real World Applications
Medical Trials
Pharmaceutical companies use binomial distribution to analyze drug effectiveness in clinical trials.
Example:
If a new medication has a 75% success rate, the probability that exactly 8 out of 10 patients improve is calculated using the binomial formula.
A vaccine is 90% effective. You vaccinate 5 people.
What is the probability that exactly 4 people develop immunity?
Step 1: Write the mathematical expression
Use the binomial formula with , , :
Sports Analytics
Coaches and analysts use binomial probability to predict game outcomes and player performance.
Example:
If a basketball player has a 70% free throw rate, binomial distribution calculates the probability of making a specific number of shots.
A soccer player scores penalty kicks 80% of the time. They take 4 penalties.
What is the probability they score exactly 3?
Step 1: Write the mathematical expression
Apply the binomial formula:
Key Takeaways
- 1The binomial distribution models successes in independent trials with success probability
- 2Formula:
- 3The combination counts the number of ways to arrange the successes
- 4Conditions: fixed , independent trials, two outcomes, constant
- 5Mean: and Standard Deviation:
Frequently Asked Questions
Glossary
- Binomial distribution
- A probability distribution for the number of successes in a fixed number of independent trials
- Trial
- A single experiment or observation with two possible outcomes
- Bernoulli trial
- An experiment with exactly two outcomes: success (probability ) or failure (probability )
- Combination
- represents the number of ways to choose items from items, regardless of order
- Independent events
- Events where the outcome of one does not affect the probability of another
Formula Card
Probability
Probability of exactly k successes in n trials
Combination
Number of ways to choose k from n
Mean
Expected number of successes
Variance
Spread of the distribution
Standard Deviation
Square root of variance