#### Week #4 – Loss Function

A loss function specifies a penalty for an incorrect estimate from a statistical model. Typical loss functions might specify the penalty as a function of the difference between the estimate and the true value, or simply as a binary value depending on whether the estimate…

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#### Week #3 – Endogenous Variable:

Endogenous variables in causal modeling are the variables with causal links (arrows) leading to them from other variables in the model. In other words, endogenous variables have explicit causes within the model. The concept of endogenous variable is fundamental in path analysis and structural equation…

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#### Week #2 – Casual Modeling

Causal modeling is aimed at advancing reasonable hypotheses about underlying causal relationships between the dependent and independent variables. Consider for example a simple linear model: y = a0 + a1 x1 + a2 x2 + e where y is the dependent variable, x1 and x2…

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#### Week #1 – Nonstationary time series

A time series x_t is called to be nonstationary if its statistical properties depend on time. The opposite concept is stationary time series . Most real world time series are nonstationary. An example of a nonstationary time series is a record of readings of the…

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#### Week #10 – Arm

In an experiment, an arm is a treatment protocol - for example, drug A, or placebo.   In medical trials, an arm corresponds to a patient group receiving a specified therapy.  The term is also relevant for bandit algorithms for web testing, where an arm consists…

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#### Week #9 – Sparse Matrix

A sparse matrix typically refers to a very large matrix of variables (features) and records (cases) in which most cells are empty or 0-valued.  An example might be a binary matrix used to power web searches - columns representing search terms and rows representing searches,…

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#### Week #8 – Homonyms department: Sample

We continue our effort to shed light on potentially confusing usage of terms in the different data science communities. In statistics, a sample is a collection of observations or records.  It is often, but not always, randomly drawn.  In matrix form, the rows are records…

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#### Week #7 – Homonyms department: Normalization

With this entry, we inaugurate a new effort to shed light on potentially confusing usage of terms in the different data science communities. In statistics and machine learning, normalization of variables means to subtract the mean and divide by the standard deviation.  When there are…

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#### Week #6 – Kolmogorov-Smirnov One-sample Test

The Kolmogorov-Smirnov one-sample test is a goodness-of-fit test, and tests whether an observed dataset is consistent with an hypothesized theoretical distribution. The test involves specifying the cumulative frequency distribution which would occur given the theoretical distribution and comparing that with the observed cumulative frequency distribution.

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#### Week #5 – Cohort Data

Cohort data records multiple observations over time for a set of individuals or units tied together by some event (say, born in the same year). See also longitudinal data and panel data.

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