Preview

Economatrics Anakysis

Powerful Essays
Open Document
Open Document
66338 Words
Grammar
Grammar
Plagiarism
Plagiarism
Writing
Writing
Score
Score
Economatrics Anakysis
ECONOMETRICS
Bruce E. Hansen c °2000, 20061 University of Wisconsin www.ssc.wisc.edu/~bhansen Revised: January 2006 Comments Welcome

This manuscript may be printed and reproduced for individual or instructional use, but may not be printed for commercial purposes.

1

Contents
1 Introduction 1.1 Economic Data . . 1.2 Observational Data 1.3 Random Sample . 1.4 Economic Data . . 1 1 1 2 2 4 4 5 6 7 8 8 9 9 10 11 11 12 14 15 15 20 21 21 22 25 26 27 28 30 32 33 34 35

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

2 Matrix Algebra 2.1 Terminology . . . . . . . . . . . . . . . . . 2.2 Matrix Multiplication . . . . . . . . . . . 2.3 Trace . . . . . . . . . . . . . . . . . . . . . 2.4 Inverse . . . . . . . . . . . . . . . . . . . . 2.5 Eigenvalues . . . . . . . . . . . . . . . . . 2.6 Rank and Positive Definiteness . . . . . . 2.7 Matrix Calculus . . . . . . . . . . . . . . . 2.8 Determinant . . . . . . . . . . . . . . . . . 2.9 Kronecker Products and the Vec Operator 3 Regression and Projection 3.1 Conditional Mean . . . . . 3.2 Regression Equation . . . 3.3 Conditional Variance . . . 3.4 Linear Regression . . . . . 3.5 Best Linear Predictor . . 3.6 Exercises . . . . . . . . . 4 Least Squares Estimation 4.1 Estimation . . . . . . . . 4.2 Least Squares . . . . . . . 4.3 Normal Regression Model 4.4 Model in Matrix Notation 4.5 Projection Matrices . . . . 4.6 Residual Regression . . . 4.7 Bias and Variance . . . . 4.8 Gauss-Markov Theorem . 4.9 Semiparametric Efficiency 4.10 Omitted Variables . . . . 4.11 Multicollinearity . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . .



Bibliography: [1] Aitken, A.C. (1935): “On least squares and linear combinations of observations,” Proceedings of the Royal Statistical Society, 55, 42-48. [2] Akaike, H. (1973): “Information theory and an extension of the maximum likelihood principle.” In B. Petroc and F. Csake, eds., Second International Symposium on Information Theory. [3] Anderson, T.W. and H. Rubin (1949): “Estimation of the parameters of a single equation in a complete system of stochastic equations,” The Annals of Mathematical Statistics, 20, 46-63. [4] Andrews, D.W.K. (1988): “Laws of large numbers for dependent non-identically distributed random variables,’ Econometric Theory, 4, 458-467. [5] Andrews, D.W.K. (1991), “Asymptotic normality of series estimators for nonparameric and semiparametric regression models,” Econometrica, 59, 307-345. [6] Andrews, D.W.K. (1993), “Tests for parameter instability and structural change with unknown change point,” Econometrica, 61, 821-8516. [7] Andrews, D.W.K. and M. Buchinsky: (2000): “A three-step method for choosing the number of bootstrap replications,” Econometrica, 68, 23-51. [8] Andrews, D.W.K. and W. Ploberger (1994): “Optimal tests when a nuisance parameter is present only under the alternative,” Econometrica, 62, 1383-1414. [9] Basmann, R. L. (1957): “A generalized classical method of linear estimation of coefficients in a structural equation,” Econometrica, 25, 77-83. [10] Bekker, P.A. (1994): “Alternative approximations to the distributions of instrumental variable estimators, Econometrica, 62, 657-681. [11] Billingsley, P. (1968): Convergence of Probability Measures. New York: Wiley. [12] Billingsley, P. (1979): Probability and Measure. New York: Wiley. [13] Bose, A. (1988): “Edgeworth correction by bootstrap in autoregressions,” Annals of Statistics, 16, 1709-1722. [14] Breusch, T.S. and A.R. Pagan (1979): “The Lagrange multiplier test and its application to model specification in econometrics,” Review of Economic Studies, 47, 239-253. [15] Brown, B.W. and W.K. Newey (2002): “GMM, efficient bootstrapping, and improved inference ,” Journal of Business and Economic Statistics. 186 [16] Carlstein, E. (1986): “The use of subseries methods for estimating the variance of a general statistic from a stationary time series,” Annals of Statistics, 14, 1171-1179. [17] Chamberlain, G. (1987): “Asymptotic efficiency in estimation with conditional moment restrictions,” Journal of Econometrics, 34, 305-334. [18] Choi, I. and P.C.B. Phillips (1992): “Asymptotic and finite sample distribution theory for IV estimators and tests in partially identified structural equations,” Journal of Econometrics, 51, 113-150. [19] Chow, G.C. (1960): “Tests of equality between sets of coefficients in two linear regressions,” Econometrica, 28, 591-603. [20] Davidson, J. (1994): Stochastic Limit Theory: An Introduction for Econometricians. Oxford: Oxford University Press. [21] Davison, A.C. and D.V. Hinkley (1997): Bootstrap Methods and their Application. Cambridge University Press. [22] Dickey, D.A. and W.A. Fuller (1979): “Distribution of the estimators for autoregressive time series with a unit root,” Journal of the American Statistical Association, 74, 427-431. [23] Donald S.G. and W.K. Newey (2001): “Choosing the number of instruments,” Econometrica, 69, 1161-1191. [24] Dufour, J.M. (1997): “Some impossibility theorems in econometrics with applications to structural and dynamic models,” Econometrica, 65, 1365-1387. [25] Efron, Bradley (1979): “Bootstrap methods: Another look at the jackknife,” Annals of Statistics, 7, 1-26. [26] Efron, Bradley (1982): The Jackknife, the Bootstrap, and Other Resampling Plans. Society for Industrial and Applied Mathematics. [27] Efron, Bradley and R.J. Tibshirani (1993): An Introduction to the Bootstrap, New York: Chapman-Hall. [28] Eicker, F. (1963): “Asymptotic normality and consistency of the least squares estimators for families of linear regressions,” Annals of Mathematical Statistics, 34, 447-456. [29] Engle, R.F. and C.W.J. Granger (1987): “Co-integration and error correction: Representation, estimation and testing,” Econometrica, 55, 251-276. [30] Frisch, R. and F. Waugh (1933): “Partial time regressions as compared with individual trends,” Econometrica, 1, 387-401. [31] Gallant, A.F. and D.W. Nychka (1987): “Seminonparametric maximum likelihood estimation,” Econometrica, 55, 363-390. [32] Gallant, A.R. and H. White (1988): A Unified Theory of Estimation and Inference for Nonlinear Dynamic Models. New York: Basil Blackwell. [33] Goldberger, Arthur S. (1991): A Course in Econometrics. Cambridge: Harvard University Press. 187 [34] Goffe, W.L., G.D. Ferrier and J. Rogers (1994): “Global optimization of statistical functions with simulated annealing,” Journal of Econometrics, 60, 65-99. [35] Gauss, K.F. (1809): “Theoria motus corporum coelestium,” in Werke, Vol. VII, 240-254. [36] Granger, C.W.J. (1969): “Investigating causal relations by econometric models and crossspectral methods,” Econometrica, 37, 424-438. [37] Granger, C.W.J. (1981): “Some properties of time series data and their use in econometric specification,” Journal of Econometrics, 16, 121-130. [38] Granger, C.W.J. and T. Teräsvirta (1993): Modelling Nonlinear Economic Relationships, Oxford University Press, Oxford. [39] Hall, A. R. (2000): “Covariance matrix estimation and the power of the overidentifying restrictions test,” Econometrica, 68, 1517-1527, [40] Hall, P. (1992): The Bootstrap and Edgeworth Expansion, New York: Springer-Verlag. [41] Hall, P. (1994): “Methodology and theory for the bootstrap,” Handbook of Econometrics, Vol. IV, eds. R.F. Engle and D.L. McFadden. New York: Elsevier Science. [42] Hall, P. and J.L. Horowitz (1996): “Bootstrap critical values for tests based on GeneralizedMethod-of-Moments estimation,” Econometrica, 64, 891-916. [43] Hahn, J. (1996): “A note on bootstrapping generalized method of moments estimators,” Econometric Theory, 12, 187-197. [44] Hansen, B.E. (1992): “Efficient estimation and testing of cointegrating vectors in the presence of deterministic trends,” Journal of Econometrics, 53, 87-121. [45] Hansen, B.E. (1996): “Inference when a nuisance parameter is not identified under the null hypothesis,” Econometrica, 64, 413-430. [46] Hansen, L.P. (1982): “Large sample properties of generalized method of moments estimators, Econometrica, 50, 1029-1054. [47] Hansen, L.P., J. Heaton, and A. Yaron (1996): “Finite sample properties of some alternative GMM estimators,” Journal of Business and Economic Statistics, 14, 262-280. [48] Hausman, J.A. (1978): “Specification tests in econometrics,” Econometrica, 46, 1251-1271. [49] Heckman, J. (1979): “Sample selection bias as a specification error,” Econometrica, 47, 153161. [50] Imbens, G.W. (1997): “One step estimators for over-identified generalized method of moments models,” Review of Economic Studies, 64, 359-383. [51] Imbens, G.W., R.H. Spady and P. Johnson (1998): “Information theoretic approaches to inference in moment condition models,” Econometrica, 66, 333-357. [52] Jarque, C.M. and A.K. Bera (1980): “Efficient tests for normality, homoskedasticity and serial independence of regression residuals, Economic Letters, 6, 255-259. 188 [53] Johansen, S. (1988): “Statistical analysis of cointegrating vectors,” Journal of Economic Dynamics and Control, 12, 231-254. [54] Johansen, S. (1991): “Estimation and hypothesis testing of cointegration vectors in the presence of linear trend,” Econometrica, 59, 1551-1580. [55] Johansen, S. (1995): Likelihood-Based Inference in Cointegrated Vector Auto-Regressive Models, Oxford University Press. [56] Johansen, S. and K. Juselius (1992): “Testing structural hypotheses in a multivariate cointegration analysis of the PPP and the UIP for the UK,” Journal of Econometrics, 53, 211-244. [57] Kitamura, Y. (2001): “Asymptotic optimality and empirical likelihood for testing moment restrictions,” Econometrica, 69, 1661-1672. [58] Kitamura, Y. and M. Stutzer (1997): “An information-theoretic alternative to generalized method of moments,” Econometrica, 65, 861-874.. [59] Koenker, Roger (2005): Quantile Regression. Cambridge University Press. [60] Kunsch, H.R. (1989): “The jackknife and the bootstrap for general stationary observations,” Annals of Statistics, 17, 1217-1241. [61] Kwiatkowski, D., P.C.B. Phillips, P. Schmidt, and Y. Shin (1992): “Testing the null hypothesis of stationarity against the alternative of a unit root: How sure are we that economic time series have a unit root?” Journal of Econometrics, 54, 159-178. [62] Lafontaine, F. and K.J. White (1986): “Obtaining any Wald statistic you want,” Economics Letters, 21, 35-40. [63] Lovell, M.C. (1963): “Seasonal adjustment of economic time series,” Journal of the American Statistical Association, 58, 993-1010. [64] MacKinnon, J.G. (1990): “Critical values for cointegration,” in Engle, R.F. and C.W. Granger (eds.) Long-Run Economic Relationships: Readings in Cointegration, Oxford, Oxford University Press. [65] MacKinnon, J.G. and H. White (1985): “Some heteroskedasticity-consistent covariance matrix estimators with improved finite sample properties,” Journal of Econometrics, 29, 305-325. [66] Magnus, J. R., and H. Neudecker (1988): Matrix Differential Calculus with Applications in Statistics and Econometrics, New York: John Wiley and Sons. [67] Muirhead, R.J. (1982): Aspects of Multivariate Statistical Theory. New York: Wiley. [68] Nelder, J. and R. Mead (1965): “A simplex method for function minimization,” Computer Journal, 7, 308-313. [69] Newey, W.K. and K.D. West (1987): “Hypothesis testing with efficient method of moments estimation,” International Economic Review, 28, 777-787. [70] Owen, Art B. (1988): “Empirical likelihood ratio confidence intervals for a single functional,” Biometrika, 75, 237-249. 189 [71] Owen, Art B. (2001): Empirical Likelihood. New York: Chapman & Hall. [72] Phillips, P.C.B. (1989): “Partially identified econometric models,” Econometric Theory, 5, 181-240. [73] Phillips, P.C.B. and S. Ouliaris (1990): “Asymptotic properties of residual based tests for cointegration,” Econometrica, 58, 165-193. [74] Politis, D.N. and J.P. Romano (1996): “The stationary bootstrap,” Journal of the American Statistical Association, 89, 1303-1313. [75] Potscher, B.M. (1991): “Effects of model selection on inference,” Econometric Theory, 7, 163-185. [76] Qin, J. and J. Lawless (1994): “Empirical likelihood and general estimating equations,” The Annals of Statistics, 22, 300-325. [77] Ramsey, J. B. (1969): “Tests for specification errors in classical linear least-squares regression analysis,” Journal of the Royal Statistical Society, Series B, 31, 350-371. [78] Rudin, W. (1987): Real and Complex Analysis, 3rd edition. New York: McGraw-Hill. [79] Said, S.E. and D.A. Dickey (1984): “Testing for unit roots in autoregressive-moving average models of unknown order,” Biometrika, 71, 599-608. [80] Shao, J. and D. Tu (1995): The Jackknife and Bootstrap. NY: Springer. [81] Sargan, J.D. (1958): “The estimation of economic relationships using instrumental variables,” Econometrica, 2 6, 393-415. [82] Sheather, S.J. and M.C. Jones (1991): “A reliable data-based bandwidth selection method for kernel density estimation, Journal of the Royal Statistical Society, Series B, 53, 683-690. [83] Shin, Y. (1994): “A residual-based test of the null of cointegration against the alternative of no cointegration,” Econometric Theory, 10, 91-115. [84] Silverman, B.W. (1986): Density Estimation for Statistics and Data Analysis. London: Chapman and Hall. [85] Sims, C.A. (1972): “Money, income and causality,” American Economic Review, 62, 540-552. [86] Sims, C.A. (1980): “Macroeconomics and reality,” Econometrica, 48, 1-48. [87] Staiger, D. and J.H. Stock (1997): “Instrumental variables regression with weak instruments,” Econometrica, 65, 557-586. [88] Stock, J.H. (1987): “Asymptotic properties of least squares estimators of cointegrating vectors,” Econometrica, 55, 1035-1056. [89] Stock, J.H. (1991): “Confidence intervals for the largest autoregressive root in U.S. macroeconomic time series,” Journal of Monetary Economics, 28, 435-460. [90] Stock, J.H. and J.H. Wright (2000): “GMM with weak identification,” Econometrica, 68, 1055-1096. 190 [91] Theil, H. (1953): “Repeated least squares applied to complete equation systems,” The Hague, Central Planning Bureau, mimeo. [92] Tobin, J. (1958): “Estimation of relationships for limited dependent variables,” Econometrica, 2 6, 24-36. [93] Wald, A. (1943): “Tests of statistical hypotheses concerning several parameters when the number of observations is large,” Transactions of the American Mathematical Society, 54, 426-482. [94] Wang, J. and E. Zivot (1998): “Inference on structural parameters in instrumental variables regression with weak instruments,” Econometrica, 66, 1389-1404. [95] White, H. (1980): “A heteroskedasticity-consistent covariance matrix estimator and a direct test for heteroskedasticity,” Econometrica, 48, 817-838. [96] White, H. (1984): Asymptotic Theory for Econometricians, Academic Press. [97] Zellner, A. (1962): “An efficient method of estimating seemingly unrelated regressions, and tests for aggregation bias,” Journal of the American Statistical Association, 57, 348-368. 191

You May Also Find These Documents Helpful

  • Better Essays

    QNT/351 Week two paper

    • 977 Words
    • 4 Pages

    McClave, J. T., Benson, P. G.,& Sincich, T. (2011). Statistics for business and economics (11th ed.).Boston, MA: Pearson-Prentice Hall…

    • 977 Words
    • 4 Pages
    Better Essays
  • Best Essays

    Lind, Marchal, and Wathen. (2008). Statistical techniques in business & economics (13th ed.). New York, NY: McGraw-Hill…

    • 2176 Words
    • 9 Pages
    Best Essays
  • Better Essays

    References: Lind, D. A., Marchal, W. G., & Wathen, S. A. (2011). Basic statistics for business and economics (7th ed.). New York, NY: McGraw-Hill/Irwin.…

    • 2042 Words
    • 9 Pages
    Better Essays
  • Satisfactory Essays

    Stat 231 Course Notes

    • 7029 Words
    • 29 Pages

    References: [1] William Kong, Stochastic Seeker : Resources, Internet: http://stochasticseeker.wordpress.com, 2011. [2] Paul Marriott, STAT 231/221 Winter 2012 Course Notes, University of Waterloo, 2009.…

    • 7029 Words
    • 29 Pages
    Satisfactory Essays
  • Powerful Essays

    BADM 528 2013 Fall Syllabus

    • 2736 Words
    • 11 Pages

    Anderson, Sweeney and Williams (2011). Essentials of Statistics for Business and Economics, 6th edition. Thomson-Southwest Publishing: ISBN: 13: 978-0-538-75457-6…

    • 2736 Words
    • 11 Pages
    Powerful Essays
  • Better Essays

    Duncan Cramer, D. H. (2004). The SAGE Dictionary of Statistics. London, England: Sage Publications. doi:: http://dx.doi.org/10.4135/9780857020123…

    • 1038 Words
    • 3 Pages
    Better Essays
  • Good Essays

    Eco 311

    • 2734 Words
    • 11 Pages

    Required readings and other materials: Introductory Econometrics, by Jeffrey M. Wooldridge (4th edition). Please note: I do not recommend that students purchase international editions or earlier editions of the textbook. Some of the material differs in these editions, including some of the end-of-chapter problems. If you have purchased an international edition or an edition earlier than the 4th, you will need to check with your classmates that you are doing the correct questions for your homework assignments. We will also analyze several scholarly articles in class. These articles can be accessed through Niihka.…

    • 2734 Words
    • 11 Pages
    Good Essays
  • Powerful Essays

    . . . . . . . . . . . . . . . . . . . . . . . .…

    • 4797 Words
    • 20 Pages
    Powerful Essays
  • Better Essays

    Qnt 351 Data Collection

    • 1346 Words
    • 6 Pages

    References: Lind, D. A., Marchal, W. G., & Wathen, S. A. (2011). Basic statistics for business and economics (7th ed.). New York, NY: McGraw-Hill/Irwin. Retrieved from the UOPX EBOOK Collection.…

    • 1346 Words
    • 6 Pages
    Better Essays
  • Powerful Essays

    Case 7.1 Anne Aylor

    • 1970 Words
    • 8 Pages

    . . . . . . . . . . . . . . . . . . . . . . .…

    • 1970 Words
    • 8 Pages
    Powerful Essays
  • Powerful Essays

    Keilegom, I. & Wilson, W. (2011). Exploring research frontiers in contemporary statistics and econometrics: A festschrift for Léopold Simar. Berlin: Springer/Physica-Verlag.…

    • 5124 Words
    • 21 Pages
    Powerful Essays
  • Powerful Essays

    wall board

    • 12820 Words
    • 52 Pages

    . . . . . . . . . . . . . . .…

    • 12820 Words
    • 52 Pages
    Powerful Essays
  • Satisfactory Essays

    . . . . . . . . . . . . . . . . . . . . . . . .…

    • 7804 Words
    • 32 Pages
    Satisfactory Essays
  • Powerful Essays

    Interest Rate Pass-Through

    • 13119 Words
    • 53 Pages

    Im, K. S., Pesaran, M. H. and Shin, Y. (2003): Testing unit roots in heterogeneous panels, Journal of Econometrics, 115, 53-74. James, C.M. and C.W. Smith, (1994): Studies in Financial Institutions: Commercial Banks. New York: McGraw Hill, (JS) Kao, C. [1999]: Spurious regression and residual-based tests for cointegration in panel data, Journal of Econometrics, 90, 1-44. Levin, A.., Lin, C-F. and Chu, C-S. J. (2002): Unit root tests in panel data: asymptotic and finite sample properties, Journal of Econometrics, 108, 1-24 Lowe, P. and T. Rohling (1992): Loan Rate Stickiness: Theory and Evidence, Research Discussion Paper, Reserve Bank of Australia. Móré, Cs. and M. Nagy (2003): Relationship between Market Structure and Bank Performance: Empirical Evidence for Central and Eastern Europe, MNB Working Paper 2003/12. Mester, L. J. and A. Saunders (1995): When does the prime rate change, Journal of Banking and Finance 19, No. 5. Mojon, B. (2000): Financial structure and the interest rate channel of ECB monetary policy, ECB Working Paper, No. 40. Pesaran, H. M., Y. Shin (1997): An autoregressive distributed lag modelling approach to cointegration analysis, in: Strom, S., P. Diamond (eds.): Centennial Volume of Ragnar Frisch, Cambridge University Press. Stiglitz-Weiss, (1981): Credit Rationing in Markets with Imperfect Information, American Economic Review, 71 (3). Sander, H. and Kleimeier, S. (2002): Asymmetric adjustment of commercial bank interest rates in the Euro area: An empirical investigation into interest rate pass-through, Kredit und Kapital, 35, 161-192. Sander, H. and Kleimeier, S. (2003): Convergence in Eurozone Retail Banking? What interest rate pass-through tells us about Monetary Policy Transmission, Competition and Integration? LIFE Working Paper No. 03-009. Szegedi, R. (2002): Kötelező jegybanki tartalékolás Magyarországon 1987-2002, (The required reserve in Hungary 1987-2002) Bankszemle 2002/6. Várhegyi, É. (2003): Bankverseny Magyarországon (Banking competition in Hungary), Közgazdasági Szemle, 12, 1027-1048. Világi, B. and Vincze, J. (1995): A kamatláb transzmissziós mechanizmus Magyarországon (1991-1995) (Interest rate transmission mechanism in Hungary (1991-1995)), Bankszemle, 1995/5. Weth, M. A. (2002): The pass-through from market interest rates to bank lending rates in Germany, Economic Research Centre of the Deutsche Bank, Discussion Paper, No. 11/02. MNB (2003): Report on financial stability http://english.mnb.hu/Engine.aspx?page=mnben_stabil&ContentID=0 (2003/2),…

    • 13119 Words
    • 53 Pages
    Powerful Essays
  • Better Essays

    Natrex

    • 14915 Words
    • 60 Pages

    Narayan, P. K., & Narayan, S. (2010). Modelling the impact of oil prices on Vietnam 's stock prices. Applied Energy, 87(1), 356–361. Ng, S., & Perron, P. (2001). LAG length selection and the construction of unit root tests with good size and power. Econometrica, 69(6), 1519–1554. Nyong, M. O., & Udah, E. B. (2012). Industrial time series of Nigeria, 1970–2009: Evolution and unit root testing in the presence of multiple endogenougs structural breaks. Applied Econometrics and International Development, 12(1). Payne, J., Lee, J., & Hofler, R. (2005). Purchasing Power Parity: Evidence from a transition economy. Journal of Policy Modeling, 27(6), 665–672. Rajan, R., & Siregar, R. Y. (2002). Choice of exchange rate regime, currency board (Hong Kong) or monitoring band (Singapore)? Australian Economic Paper, 41(4), 538–556. Selaive, J., & Tuesta, V. (2003). Net foreign assets intensity and imperfect financial integration: An empirical approach. Central Bank of Chile Working Papers, No 252. Shen, L. (1999). China macroeconomic annual model. In H. Wang, B. Li, & S. Li (Eds.), China available macroeconomic models. Beijing: China Finance and Economics Publishing House. Shen, L. (2000). China 's macro econometric annual model. In L. R. Klein, & S. Ichimura (Eds.), Econometric modelling of China. Singapore: River Edge; New Jersey: World Scientific Publishing Co. Pte. Ltd. Shi, J. (2006). Are currency appreciations concretionary in China. NBER Working Paper, No 12551. Song, H., Liu, Z., & Jiang, P. (2001). Analysing the determinants of China 's aggregate investment in the reform period. China Economic Review, 12, 227–242.…

    • 14915 Words
    • 60 Pages
    Better Essays