THE large increase in the labor force participation of women during recent times has been accompanied by a less-noted but also important change in their employment picture-the growth of part-week work.' While the participation rate of women was increasing from 37% in 1957 to 49% in 1977, the percentage of female employment in part-week jobs was rising from 20% to 27% (U.S. Office of the President, 1970, 1978).2 Economists have linked these changes to both the secular rise of the unemployment rate in the United States and the differential between female and male unemployment (Friedman, 1977; Niemi, 1974; Perry, 1970). For example, Friedman states

Q UALITY of service has been neglected in empirical studies of air travel. Travel demand depends on travel time as well as the usual price and activity variables. Travel time includes not only average en route time, but delay. Following Douglas-Miller (1974a,b), frequency delay is the gap between one's desired and the nearest offered departure time, while stochastic delay is time lost due to the nearest offered departure being unavailable. Their sum, schedule delay, is the major element in air service quality. Despite its theoretical importance, no empirical study of air travel demand has incorporated it.' This one does.2 A second novel feature of this study is that it models demand on a service segment rather than passenger origin-destination demand. From the carriers' point of view, this is the relevant market demand. Much air travel is done in several segments rather than non-stop. Ideally, segment demand should be modelled based on passenger origin-destination demand, with service segment demand built up from all the possible routings using the segment under study. In practice this is impossible, since the origin-destination passenger data contain no information on routings. This is why previous demand studies, based on origin-destination data, have been unable to incorporate consideration of delay. The third novel feature of this study is that it is based on time series estimation embedded in a simultaneous equations model of service segment markets. Elsewhere (Anderson-Kraus, 1980), we have reported on the supply side of the model. The time chosen is 1973-76 (48 monthly observations), when fares were set according to the CAB formula, and flight scheduling rivalry dominated carrier competition. The results below indicate that segment demand can be successfully modelled with these methods. It is notable that price elasticities in excess of one in absolute value are found even in a number of business markets. It is dangerous to draw general conclusions from a specific instance of success, but the results do support further attempts to model demand incorporating delaybased quality of service. Many markets are characterized by significant waiting costs and competition over the reduction of these costs. Where feasible, the results below indicate that even crude measures of delay have a significant payoff. Part I lays out the model of demand and service quality and embeds it in the model of market equilibrium. Part II presents the results.

A potentially important parameter that has not yet been convincingly estimated is the critical concentration ratio. This paper attempts to estimate it in three types of market. The estimation of a critical concentration ratio, if one exists, seems of potentially great importance because of its implication for antitrust policy. If a critical concentration ratio were found and if concentration had no effect below that level, it would seem to follow that a horizontal merger in a market where concentration was below the critical level and where the merger could not increase concentration to the critical level could not substantially lessen competition or tend to create monopoly.

T HE relationship between direct investment by U.S. firms and the decline in U.S. export trade shares has been a subject of bitter controversy for at least the last twenty years. These changes over time in trade flows have undoubtedly been influenced by other factors such as productivity and monetary and fiscal policy in the United States and elsewhere. All of these were presumably reflected in price and income changes, the effects of which on trade probably have swamped any that might have stemmed from changes in the level of direct investment. One way to disentangle some of these influences might be to disaggregate by country, industry, and even better, by firm. However, we have not yet developed enough disaggregated time series data for this purpose and have therefore chosen to work first with cross-sections by country and industry. The method used here can be summarized by saying that we examine exports to a crosssection of 44 foreign destinations in 1970 by the United States and by 13 other major exporting countries. We relate these exports to characteristics of the destinations and to production in them by affiliates of companies based in the United States and those based in other countries. The destination country characteristics are their size (measured by their Gross Domestic Product (GDP), converted to dollars by exchange rates, or their total imports of manufactures), their membership in the European Economic Community (EEC), and their distance from the United States and from Germany (representing distance from most of the other major exporters). The affiliate activity variables are measures of the output of U.S.-owned manufacturing and nonmanufacturing affiliates and of the number of foreign-owned manufacturing affiliates in each country.' The measures for U.S.-owned affiliates were net sales (total sales less imports from the United States), and estimated net local sales (net sales multiplied by the ratio of local sales to total sales of the affiliate). In effect we have taken the elements of a fairly crude standard trade model (country size, distance, and membership in a trade bloc)2 and added to that some variables describing direct investment by the United States and other countries, to ask whether the latter have any impact on trade beyond that of the country characteristics. The basic assumption implied by this analysis is that the answer to the question What would have happened if U.S. or foreign companies had not invested in a country or had invested less? is provided by trade with those countries in which U.S. or foreign firms did not invest, or invested less than in others. The danger in this assumption is obvious: that there are factors which simultaneously affect investment and trade, giving a spurious appearance of a relationship between them. Such spurious relationships could work in the direction of showing that investment either increases or decreases trade. Size of host country, if it were omitted, would work in the former direction of suggesting complementarity between investment and trade, Received for publication October 2, 1980. Accepted for publication October 21, 1980. * National Bureau of Economic Research and Queens College, City University of New York, and National Bureau of Economic Research and Temple University, respectively. The research reported on in this paper was financed by grants to the National Bureau of Economic Research from the National Science Foundation and the Ford Foundation. However, it is not an official National Bureau publication. In particular, it has not been submitted to the Board of Directors for approval. Any opinions, findings, conclusions or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the sponsoring or financing organizations. Mary Boger, Linda Quandt, and Marianne Rey were responsible for data collection and organization and Muriel Moeller for preparation of the manuscript. We are indebted to the Bureau of Economic Analysis of the U.S. Department of Commerce for the use of their data and particularly to Arnold Gilbert and Michael Liliestedt of the BEA for programming and other assistance with these data. Earlier versions of this paper appeared as NBER Working Papers 87 and 131. I No data were available on foreign-owned nonmanufacturing affiliates or on the size of foreign-owned manufacturing affiliates. 2 See, for example, Leamer and Stern (1970), Linneman (1966), Taplin (1967), and Tinbergen (1962).

of economic time series similar to the observed high frequency series. A transformation that converts these series to serially uncorrelated stationary time series would therefore introduce the same conversion to the residuals. Although the discussion in this paper has been limited to the problem of distribution, a similar treatment can be given to the problem of interpolation and extrapolation by related series. As has been shown in the paper of Chow-Lin, the three problems can be treated simultaneously by properly defining the transformation matrix B.