1Probability
The ratio of the number of ways an event can occur to the total number of possible outcomes.
2Event
An outcome or group of outcomes of an experiment.
3Compound Event
An event involving the simultaneous occurrence of two or more events.
4Independent Events
Events in which the occurrence of one does not affect the probability of another.
5Random Error
An unpredictable measurement error that may be positive or negative.
6Normal Distribution
A bell-shaped probability distribution commonly used to describe random measurement errors.
7Probable Error
A range around the mean within which a specified percentage of observations is expected to fall.
8P(E) = Number of favorable outcomes / Total number of possible outcomes
Basic Probability Formula:
9• Probability of occurrence: P(E) = m / (m + n)
• Probability of failure: P(not E) = n / (m + n)
If an event can occur in m ways and fail to occur in n ways:
100 to 1
Probability ranges from
11P(E) = 0
means the event is impossible.
12P(E) = 1
means the event is certain.
131
The sum of all possible outcome probabilities equals
14Compound Event
The simultaneous occurrence of two or more events.
15P = P₁ × P₂ × P₃ × ... × Pₙ
Where:
• P — Probability of simultaneous occurrence.
• P₁, P₂, P₃, ... — Individual event probabilities.
• n — Number of independent events.
Probability of Compound Events
16• +1 — Positive error.
• −1 — Negative error.
Since the error is random, there are two possible values:
17T = 2ⁿ
Where:
• T — Total number of possible combinations.
• n — Number of combined measurements.
Total Number of Possibilities:
18distribution of random errors;
bell-shaped
As the number of combined measurements increases, the ___ approaches a normal distribution, which has a ___ curve.
19Normal Distribution Curve
A bell-shaped curve representing the distribution of random errors.
20Probability Density Function (PDF)
A mathematical function describing the distribution of probabilities.
21Standard Deviation (σ)
Measures the spread of observations around the mean.
22Mean (μ)
The central or average value of a distribution.
23f(x) = [1 / (σ × √(2π))] × e^[−(x − μ)² / (2σ²)]
Where:
• f(x) — Probability density function.
• e — Base of natural logarithms (approximately 2.71828).
• x — Value of the random variable or error.
• μ — Population mean.
• σ — Population standard deviation.
• π — Approximately 3.14159.
Normal Probability Density Function:
24f(x) = [1 / (σ × √(2π))] × e^[−x² / (2σ²)]
For a Normal Distribution with Mean Zero:
25f(μ) = 1 / (σ × √(2π))
Central Ordinate:
26• Mean (μ) = 0
• Variance (σ²) = 1
• Standard deviation (σ) = 1
For a standard normal distribution:
27Standard Normal Distribution
A normal distribution with a mean of 0 and a standard deviation of 1.
28Z-Value (Standard Score)
Indicates how many standard deviations a value is from the mean.
29Standard Normal Distribution Table
A table used to determine the probability or area under the normal curve.
30z = (x − μ) / σ
Where:
• z — Standard score.
• x — Observed value.
• μ — Mean.
• σ — Standard deviation.
Formula for the Z-Value:
31z = (x − μ) / σ
Converting a Normal Distribution into a Standard Normal Distribution:
32P(Z ≤ z) = Φ(z)
• Φ(z) — Area under the standard normal curve from negative infinity up to z.
Standard Normal Cumulative Probability:
33standard normal distribution table
provides cumulative probabilities.
34P(Z > z) = 1 − Φ(z)
To Find the Probability Above a Z-Value:
35P(a < Z < b) = Φ(b) − Φ(a)
Where:
• a — Lower z-value.
• b — Upper z-value.
• Φ(b) — Cumulative probability at the upper limit.
• Φ(a) — Cumulative probability at the lower limit.
Probability Between Two Values
36normal distribution
is symmetrical around its mean.
37Φ(−z) = 1 − Φ(z)
Symmetry of the Normal Distribution
38P(−z < Z < +z) = 2Φ(z) − 1
Probability Between Equal Positive and Negative Z-Values:
3968.3%
For a normal distribution, approximately ___ of observations fall within one standard deviation above or below the mean.
40±1σ
Approximately 68.3% of observations fall within this range.
41±2σ
Approximately 95.45% of observations fall within this range.
42±3σ
Approximately 99.73% of observations fall within this range.
43Percent Probable Error
The error range around the mean corresponding to a specified probability percentage.
44Eₚ = k × S
Where:
• Eₚ — Probable error at a specified percentage.
• k — Multiplier corresponding to the desired probability.
• S — Standard deviation of the observations.
Percent Probable Error
45x̄ = Σx / n
Arithmetic Mean
46S = √[Σ(x − x̄)² / (n − 1)]
Sample Standard Deviation
47E₅₀ = 0.6745S
50% Probable Error
48E₉₅ = 1.960S
95% Probable Error
49• 50% range: x̄ ± E₅₀
• 95% range: x̄ ± E₉₅
Acceptable Ranges
50Percentage = (Number of observations within the range / Total observations) × 100
Percentage of Observations Within a Range