A received total wideband power (61A) in a wireless communications system is measured a number of times during a time interval. Preferably, also code powers ((C/I)i) are measured, directly or indirectly. A probability distribution of a first power quantity (63A, 63B), typically total received power is estimated from the set of measurements, preferably using Kalman filtering techniques, and preferably also a probability distribution of a second power quantity (62A, 62B) related to noise and interference signals. A conditional probability distribution of a noise rise measure to be determined is based at least on the probability distribution of the total power (63A, 63B). From that conditional probability distribution, a value of the noise rise measure is calculated. In a preferred embodiment, the noise rise measure is based on a quotient between total power and a noise floor, and the conditional probability distribution of the noise rise is therefore based on a conditional probability distribution of noise floor quantity (64). The conditional probability distribution of the noise floor quantity (64) is in turn based on the probability distribution of the extreme value of a second power quantity (62A, 62B) related to noise and interference signals and a prior determined probability distribution of a noise power floor.