A cyclic time-dependent Markov process to model daily patterns in wind turbine power production

Wind energy is becoming a top contributor to the renewable energy mix, which raises potential reliability issues for the grid due to the fluctuating nature of its source. To achieve adequate reserve commitment and to promote market participation, it is necessary to provide models that can capture daily patterns in wind power production. This paper presents a cyclic inhomogeneous Markov process, which is based on a three-dimensional state-space (wind power, speed and direction). Each time-dependent transition probability is expressed as a Bernstein polynomial. The model parameters are estimated by solving a constrained optimization problem: The objective function combines two maximum likelihood estimators, one to ensure that the Markov process long-term behavior reproduces the data accurately and another to capture daily fluctuations. A convex formulation for the overall optimization problem is presented and its applicability demonstrated through the analysis of a case-study. The proposed model is capable of reproducing the diurnal patterns of a three-year dataset collected from a wind turbine located in a mountainous region in Portugal. In addition, it is shown how to compute persistence statistics directly from the Markov process transition matrices. Based on the case-study, the power production persistence through the daily cycle is analysed and discussed

Measuring Employment and Income for Low-Income Populations with Administrative and Survey Data

We discuss the strengths and weaknesses of income and employment data in national surveys, in unemployment insurance (UI) wage records, and in tax returns. The CPS, SIPP, NLS, and PSID surveys provide valuable information on the behavior of the low-income population. They have broad and fairly accurate measures of income for national samples, and their focus on families as the unit of analysis and their ease of access greatly enhance their value. The value of these data sets for evaluating welfare reform is severely limited, however. With the devolution of responsibility for TANF, the CPS and SIPP sampling frames and sample sizes mean that, at best, they can be only supplementary data sources for understanding the effects of welfare reform at the state and local levels. The apparent decline in program coverage in the CPS is also worrisome. UI data are available at the state level and can be matched to individuals in existing samples at relatively low cost. It is straightforward to do follow-up analyses on income and employment for workers who remain in the state, and UI data are timely. However, earnings are available only for individuals, while changes in family composition upon exit from welfare have been shown to have a large bearing on economic well-being. UI data do not allow us to track these changes. There also appears to be a substantial problem with some workers being classified as independent contractors and hence not entering the UI system. Overall gaps in coverage appear to be at least 13 percent and may be significantly higher. Even when wages are reported, there is some evidence that they are understated by a significant amount. We also present evidence on the degree to which tax data can be used to understand the incomes and employment of low-skilled workers. The paper concludes with brief recommendations for future research that might help fill some of the gaps we have identified.

The Earned Income Tax Credit

Since its inception in 1975, the Earned Income Tax Credit (EITC) has grown into the largest, Federally-funded means-tested cash assistance program in the United States. In this chapter, we review the political history of the EITC, its rules and goals and provide a broad set of program statistics on its growth and coverage. We summarize conceptual underpinnings of much of the recent economic research on the EITC, discussing participation in the credit and compliance with its provisions, and its effects on labor force participation and hours of work, marriage and fertility, skill formation and consumption. We note that participation rates of the credit are high, rates of credit noncompliance are also high, and that there are theoretical reasons to prefer the EITC to other anti-poverty programs if one's objective is to encourage work among the poor. We also note that the predicted effects of the EITC are not all pro-work, especially with respect to hours and its labor market incentives for two-earner couples. We then summarize the existing empirical research on the behavioral effects of the EITC, paying particularly emphasis to the effects of the 1986, 1990 and 1993 expansions of the credit on labor force participation and hours of work. The literature provides consistent evidence, generated from a variety of empirical approaches, that the EITC positively affects labor force participation. The literature also finds smaller, negative effects on hours of work for people already in the labor market and for secondary workers. We conclude the chapter with a discussion of the ongoing EITC-related policy debates and highlight what, if any, critical economic issues underlie these debates.

Examining the Effect of the Earned Income Tax Credit on the Labor Market Participation of Families on Welfare

This paper examines the employment effects of the earned income tax credit (EITC). We use a unique dataset, created by matching administrative data from public assistance records, unemployment insurance records, and federal tax returns for a sample of California residents. We conduct a set of four tests to assess our ability to isolate the causal effects of the EITC on employment. The first test is based on the intuition that if the EITC alters employment, all else being equal, employment rates for two-or-more child families should grow relative to the employment rates of one-child families, as credit amounts available to these groups of families diverged over the 1990s. The second test examines whether or not people eligible for the EITC actually file tax returns and claim it. The third test is based on the intuition that, if the EITC, and not other factors such as the strong economy in the 1990s, is causing employment differences between families with two or more children relative to those with one child, we should expect to see no employment differences (after conditioning on other characteristics) between families with two children and families with three or more children, since the EITC did not change differentially for the latter two groups. The fourth test conditions the sample on those who do not file tax returns and again examines employment changes in the 1990s for families with two or more children relative to families with one child. Using fixed-effects empirical employment models estimated on a sample of single-parent families, our coefficient estimates are consistent with the EITC having a substantial, positive effect on the employment of families who have used or will use welfare.