A method for estimating playout resolution of a video delivered to a client device by an encrypted video stream communicated over a network, the method comprising: (i) obtaining a first probability distribution for a plurality of states, each of the plurality of states comprising a different level of playout resolution that may be achieved at the client device by the delivered video stream, and wherein the first probability distribution indicates either an initial estimate of the probability for each of the plurality of states or a probability for the state in a time-varying Markov chain that depends upon an immediately preceding state delivered by the video stream and a respective time-varying state transition probability; (ii) selecting a current chunk of the encrypted video stream expected to comprise data packets carrying video data of the same level of playout resolution; (iii) by analysis of the data packets in the selected current chunk, determining values for features in a predetermined set of features indicative of conditions in the network; (iv) using a pre-generated model to obtain, given the determined set of feature values from (iii), a corresponding set of time-varying state transition probabilities for the plurality of states; (v) applying the determined set of time-varying state transition probabilities from (iv) to calculate, using the first probability distribution obtained at (i), a second probability distribution for the plurality of states of the Markov chain; and (vi) using the second probability distribution to estimate which of the plurality of different levels of playout resolution will be delivered to the client device by the current chunk of the encrypted video stream. (i) obtaining a first probability distribution for a plurality of states, each of the plurality of states comprising a different level of playout resolution that may be achieved at the client device by the delivered video stream, and wherein the first probability distribution indicates either an initial estimate of the probability for each of the plurality of states or a probability for the state in a time-varying Markov chain that depends upon an immediately preceding state delivered by the video stream and a respective time-varying state transition probability; (ii) selecting a current chunk of the encrypted video stream expected to comprise data packets carrying video data of the same level of playout resolution; (iii) by analysis of the data packets in the selected current chunk, determining values for features in a predetermined set of features indicative of conditions in the network; (iv) using a pre-generated model to obtain, given the determined set of feature values from (iii), a corresponding set of time-varying state transition probabilities for the plurality of states; (v) applying the determined set of time-varying state transition probabilities from (iv) to calculate, using the first probability distribution obtained at (i), a second probability distribution for the plurality of states of the Markov chain; and (vi) using the second probability distribution to estimate which of the plurality of different levels of playout resolution will be delivered to the client device by the current chunk of the encrypted video stream.