Hhat

Hhat is the plain-text rendering of the mathematical notation h-hat, which is commonly used in statistics and related fields to denote an estimator of the quantity h. The hat symbol over a variable, often written in typeset form as ĥ, signals that the value is estimated from data rather than observed directly. In environments that do not support diacritical marks, authors frequently render this as hhat or h_hat.

In statistical notation, ĥ may represent any estimated parameter, function, or statistic named h. It is used across disciplines such as econometrics, epidemiology, and signal processing to distinguish an estimate from the true underlying parameter h. The specific meaning of ĥ depends on the model and context; for example, h might be a hazard function, a treatment effect, a bandwidth in kernel density estimation, or a regression coefficient, with ĥ denoting its estimate.

In programming and data science workflows, hhat can appear as a variable name following common coding conventions for vectors and scalars. It may be assigned as the result of an estimation procedure, prediction model, or inferential calculation.

Other uses of the string hhat include its role as a username, product name, or file identifier in various software projects. However, there is no universally defined concept named hhat outside of its role as a textual render of ĥ.

See also: hat (mathematics), estimator, h-hat, h_hat.