X.mat is a 70*1 cell array which contains 70 signals at different degradation level (from low to high). Please use time domain method (mean, mean square, peak, skewness, kurtosis) to extract the features of degradation. And find the best feature using metrics including monotonicity, correlation coefficient and percentage mean deviation.

In this question, the feature is a single value for each signal. You will get 70 values in total for every kind of feature, and you may see the feature value change with the degradation level (time). The monotonicity/correlation coefficient/percentage mean deviation are metrics that can evaluate the relationship between the feature values and degradation level (time). And these metrics should be a single value for each feature.