12. (a) In the context of descriptive statistics, explain briefly the following concepts: (i) mean, (ii) median, (iii) mode, (iv) variance, (v) standard deviation, (vi) covariance, (vii) correlation coefficient, (viii) regression coefficient. (b) Explain which of the concepts (i)-(ix) make statistical sense when the data in question are measured on different scales: nominal, ordinal, interval, and ratio. 13. Compare and contrast time-series, cross-section, and panel data as they relate to heterogeneity and dependence. 14. Explain how the different features of observed data can be formalized in the context of expressing a data series in the form {xk, xk Є Rx, k ≤ N}. 15. Explain briefly the connection between chance regularity patterns and probability theory concepts. 16. Explain the connection between chance regularities and statistical models. 17. Explain the notion of statistical adequacy and discuss its importance for statistical inference.