Combine density probabilities moments and model interpretation
Connect supported event areas, reliability thresholds and transformed moments while keeping model limits explicit.
Exam scope and assessment notes
Probability & Statistics 2 (Paper 6, A Level) · Mathematics 9709, 2026–2027, section 6.3. Paper 6 builds on Paper 5 and assumes Paper 3 calculus. A probability density is non-negative and has total area one over a single interval; its support may be infinite. Density height is not probability, and individual points have probability zero. Calculate interval probabilities, median and other percentiles directly from density areas. Explicit cumulative distribution function knowledge is not required by this section. Calculate E(X) and E(X²) by integration, then Var(X) = E(X²) − E(X)² when the needed moments are finite. Infinite-support densities require convergent improper integrals; a valid density need not have finite moments. Only single-interval densities are used here, with zero density outside the stated support. Model conclusions are conditional on the stipulated distribution; examples do not establish a real population model. Sampling, estimation and hypothesis testing belong to later sections. Written practice is self-checked; cumulative assessment and human subject-teacher review remain pending.
Before you begin
- Calculate moments and percentiles.
- Use negative affine transforms and reverse inequalities.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How is the support of 10 − 2X obtained from 0 ≤ X ≤ 2?
What you will learn
- Solve a multi-step density application.
- Translate a reflected event and assess a model claim.
Build a supported model first
A stipulated duration X in minutes has density kx² on [0,2]. Normalisation gives k = 3/8. Its mean is 3/2 minutes, second moment 12/5 minutes squared and variance 3/20 minutes squared. P(X > 1) = 7/8.
For a threshold covering 90% of model durations, integrate from zero to t: t³/8 = 0.9. Thus t = 2(0.9)^(1/3) ≈ 1.9310 minutes. This is a probability threshold, not a guarantee that every duration is below it.
Translate the whole event
If U = 10 − 2X, support becomes [6,10], mean becomes seven and variance becomes 3/5. The event U > 8 means X < 1, since dividing by a negative coefficient reverses the direction. Its probability is 1/8, not 7/8.
A real observed duration above two contradicts this exact bounded support model for that population. A finite sample lying within [0,2] does not prove the density shape. Model checking requires relevant data and a defined measurement process; this example is teaching content, not empirical evidence.
Pause and explain
Which original event corresponds to 10 − 2X > 8?
Worked example
Normalise kx² on [0,2] and calculate mean and variance.
Show the worked solution
- 8k/3 = 1, so k = 3/8.
- Mean = (3/8)[x⁴/4]₀² = 3/2.
- Second moment = (3/8)[x⁵/5]₀² = 12/5.
- Variance = 12/5 − 9/4 = 3/20.
Answer k = 3/8; mean 3/2; variance 3/20.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Normalise kx² on [0,2], then calculate mean and variance.
Give me a hint
Use full area and two weighted integrals.
Compare my reasoning
- k = 3/8.
- Mean = 3/2.
- Second moment = 12/5.
- Variance = 3/20.
k = 3/8; mean 3/2; variance 3/20.
Look for these in your work
- Normalise first.
- Use exact moments.
Find a threshold t with P(X ≤ t) = 0.9 and interpret it.
Give me a hint
Integrate the density from zero to t and set its area t³/8 to 0.9.
Compare my reasoning
- t = 2(0.9)^(1/3).
- t ≈ 1.9310 minutes.
- Ten percent of model durations exceed it.
t ≈ 1.9310 minutes; it is not a guaranteed maximum.
Look for these in your work
- Solve inside support.
- State probability interpretation.
For U = 10 − 2X, find support, mean, variance and P(U > 8).
Give me a hint
Transform endpoints and moments, then reverse the event inequality.
Compare my reasoning
- Support [6,10]; mean seven.
- Variance = 4(3/20) = 3/5.
- U > 8 means X < 1.
- Probability is 1/8.
Support [6,10]; mean 7; variance 3/5; probability 1/8.
Look for these in your work
- Reverse and reorder.
- Keep model assumptions explicit.
Common mistakes
- A probability threshold is not a guarantee.
- Reverse a reflected inequality.
- A stipulated model is not proof about real data.
What can you explain now?
Does one observation prove this density?
Compare with the explanation
No.
Model shape and support need evidence beyond the example.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Normalise, integrate the exact event, check moments and interpret what the model actually claims.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.