# skfolio.utils.tools.half_life_to_decay_factor ### skfolio.utils.tools.half_life_to_decay_factor(half_life) Convert half-life to exponential decay factor. The decay factor ($\lambda$) determines how much weight is given to past observations in exponentially weighted calculations. It is computed from the half-life using: $$ \lambda = 2^{-1/\text{half-life}} $$ * **Parameters:** **half_life** *float* : Half-life in number of observations. This is the number of observations for the weight to decay to 50%. Must be positive. * **Returns:** **decay_factor** *float* : The exponential decay factor ($\lambda$), satisfying $0 < \lambda < 1$. ### Examples ```pycon >>> half_life_to_decay_factor(40) 0.9828... ```