Due to the conditional dependencies between states at different time points, calculation of the likelihood of time series data is somewhat tedious, which illustrates the motivation to use ABC. A computational issue for basic ABC is the large dimensionality of the data in an application like this. The dimensionality can be reduced using the summary statistic formula_48, which is the frequency of switches between the two states. The absolute difference is used as a distance measure formula_49 with tolerance formula_50. The posterior inference about the parameter formula_1 can be done following the five steps presented in.