Recommended anomaly detection technique for simple, one-dimensional scenario?

Question

I have a scenario where I have several thousand instances of data. The data itself is represented as a single integer value. I want to be able to detect when an instance is

Answer 1

Check out the three-sigma rule:

mu  = mean of the data
std = standard deviation of the data
IF abs(x-mu) > 3*std  THEN  x is outlier

An alternative method is the IQR outlier test:

Q25 = 25th_percentile
Q75 = 75th_percentile
IQR = Q75 - Q25         // inter-quartile range
IF (x < Q25 - 1.5*IQR) OR (Q75 + 1.5*IQR < x) THEN  x is a mild outlier
IF (x < Q25 - 3.0*IQR) OR (Q75 + 3.0*IQR < x) THEN  x is an extreme outlier

this test is usually employed by Box plots (indicated by the whiskers):

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EDIT:

For your case (simple 1D univariate data), I think my first answer is well suited. That however isn't applicable to multivariate data.

@smaclell suggested using K-means to find the outliers. Beside the fact that it is mainly a clustering algorithm (not really an outlier detection technique), the problem with k-means is that it requires knowing in advance a good value for the number of clusters K.

A better suited technique is the DBSCAN: a density-based clustering algorithm. Basically it grows regions with sufficiently high density into clusters which will be maximal set of density-connected points.

DBSCAN requires two parameters: epsilon and minPoints. It starts with an arbitrary point that has not been visited. It then finds all the neighbor points within distance epsilon of the starting point.

If the number of neighbors is greater than or equal to minPoints, a cluster is formed. The starting point and its neighbors are added to this cluster and the starting point is marked as visited. The algorithm then repeats the evaluation process for all the neighbors recursively.

If the number of neighbors is less than minPoints, the point is marked as noise.

If a cluster is fully expanded (all points within reach are visited) then the algorithm proceeds to iterate through the remaining unvisited points until they are depleted.

Finally the set of all points marked as noise are considered outliers.