Valeurs aberrantes et leur signification

Sometimes one value sits far from the rest of the pack. That lonely point is an outlier, and it can quietly distort your whole analysis. 🚩

A single extreme value can yank the mean and inflate the standard deviation. Spotting outliers early stops them from hiding the real pattern.

Not every outlier is junk. Some are typos to fix, but others are a genuine signal like fraud or a rare event worth your full attention.

The classic test uses the IQR. Anything far below Q1 or far above Q3 is flagged as a likely outlier worth a second look.

Build a fence at Q1 minus 1.5 times IQR and Q3 plus 1.5 times IQR. Points outside this fence are the suspected outliers.

A boolean mask finds them fast. Compare your column to the fences and pandas returns exactly the rows that fall outside.

Another approach is the z-score, how many standard deviations a point sits from the mean. Values beyond about three are often treated as outliers.

A box plot draws the IQR fence for you and marks anything past it as separate dots. It is the fastest visual outlier scan you have.

Never drop a point just because it is extreme. Investigate it first, because removing real data can erase the very insight you needed.

Once understood, you can remove, cap, or keep an outlier. Capping values at the fence is a gentle middle ground that limits the damage.

Whatever you decide, write it down. A clear note on which outliers you changed and why keeps your analysis honest and reproducible.

You spot a value sitting far above Q3 in your dataset.

You can now detect outliers with the IQR rule or z-scores, judge whether they are error or signal, and handle them on purpose. 🎓