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A value below which a given percentage of observations in a dataset fall.
DESCRIPTION
Percentiles summarize distributions by indicating thresholds at specified percentages (e.g., 95th percentile). They are widely used for SLAs, anomaly thresholds, and robust summaries. Different interpolation methods can yield slightly different percentile values. Box plots visualize quartiles and extremes. In performance engineering, high percentiles capture tail latencies. Context and sample size affect interpretation.
ALTERNATIVE DEFINITIONS
Wikipedia
A percentile is a measure used in statistics indicating the value below which a given percentage of observations in a group of observations falls.
Standard definition emphasizing threshold interpretation.
NIST/SEMATECH e-Handbook
The pth percentile is a value such that p percent of the observations fall below it.
Concise definition used in quality and reliability contexts.
RELATED TERMS
Quartile Median Interquartile Range Cumulative Distribution Function Box Plot Histogram Outliers Trimmed Mean
COMPETING TERMS
KNOWLEDGE AREAS
Mathematics Data Science Data Visualization
TARGET AUDIENCE
Data Analyst BI Specialist Data Scientist
EXTERNAL SOURCES
- Wikipedia - Percentile
- NumPy - Percentile function and interpolation options
- R stats - R quantile function with type variants
- Google SRE Book - Use of high percentiles for SLIs
TOP QUESTIONS
Why use Percentiles instead of averages for performance?
Percentiles capture tail behavior that averages can hide, better reflecting user experience.
Category: Relevancy
What is the difference between Percentile and Quantile?
Percentiles are quantiles expressed as percentages; quantiles generalize to any fraction p in [0,1].
Category: Misunderstanding
Why do different tools yield slightly different Percentiles?
They use different interpolation or ranking methods. Specify and document the chosen definition.
Category: Mistakes
How many samples are needed for reliable high Percentiles?
High percentiles, like 99th, require large samples for stability; small samples produce noisy estimates.
Category: Requirements
What is a P95 SLO?
A service level objective stating 95% of requests must be faster than a threshold value.
Category: Definition
How do Percentiles relate to box plots?
Box plots visualize median (50th), quartiles (25th, 75th), and whiskers often derived from percentiles.
Category: Example
When should I prefer robust percentiles?
In presence of heavy tails or outliers, percentiles summarize distribution more robustly than mean.
Category: Opinion
How to compute streaming Percentiles?
Use approximate algorithms like t-digest or HDR histogram for large-scale streaming data.
Category: Application
What’s a common misunderstanding about Percentiles in SLAs?
Believing a 95th percentile target guarantees all requests meet the threshold; 5% can be slower.
Category: Misunderstanding