100 PHILIPPINE JOURNAL OF DEVELOPMENTstatistical significance of labor rises, while that of fertilizer falls. The dummyvariable for tractor use rema<strong>in</strong>s statistically <strong>in</strong>significant. Results providestatistical support for the existence of a stochastic frontier. Follow<strong>in</strong>g Schmidtand L<strong>in</strong> (1984), we use the square root of the variance ratio as the basis fortest<strong>in</strong>g the existence of a frontier <strong>in</strong> Model 2. In our case the estimated value( λ = 4.12, t = 5.27) is significantly different <strong>from</strong> zero at the 95 percentconfidence level.Model 3 is a frontier model estimated us<strong>in</strong>g parcel-level random effects.In Model 3 all po<strong>in</strong>t estimates are significantly different <strong>from</strong> zero at the 95percent confidence level, with the exception of the dummy variables foryears and tractor use. Coefficients on the year dummy variables suggest thatyields were somewhat lower <strong>in</strong> 1999 than <strong>in</strong> 1995 or 1997. As <strong>in</strong> previousmodels, yields on parcels <strong>in</strong> site 2 (Tamlang) were lower on average than onthose <strong>in</strong> site 1 (Marangas). Aga<strong>in</strong>, yields were much higher <strong>in</strong> the ra<strong>in</strong>yseason and approximately 52 percent higher <strong>in</strong> the dry season <strong>in</strong> the presenceof irrigation. A Lagrange multiplier test provides statistical evidence thatsupports the parcel-level random effects specification. For this test, we comparepanel and nonpanel regressions <strong>in</strong> the absence of a frontier. The Lagrangemultiplier test statistic is distributed with one degree of freedom. The2test statistic is 5.38, which exceeds the χ critical value of 3.84. We thereforeconclude that, <strong>in</strong> this case, an <strong>in</strong>dividual effects model is preferred to amodel without <strong>in</strong>dividual effects at a 95 percent significance level. We notethat the coefficients on labor, fertilizer, and pesticide are positive andsignificant <strong>in</strong> Model 3 and suggest dim<strong>in</strong>ish<strong>in</strong>g returns to use of these <strong>in</strong>puts.Yields appear to be most sensitive to fertilizer application, followed by levelsof labor and pesticides, respectively.In Model 4 we relax the assumption of time-<strong>in</strong>variant productioncoefficients <strong>in</strong> order to assess the extent to which factor productivity changedover the sample period. For each <strong>in</strong>put (labor, fertilizer, and pesticide), Model4 <strong>in</strong>troduces two additional regression coefficients <strong>in</strong> the form of <strong>in</strong>putdummy<strong>in</strong>teraction terms for 1995 and 1997. For each <strong>in</strong>put, the nullhypothesis of no change <strong>in</strong> <strong>in</strong>put elasticities can be exam<strong>in</strong>ed via a Wald test.We test the assumption that the <strong>in</strong>put-year <strong>in</strong>teraction terms are jo<strong>in</strong>tly equalto zero, i.e., that the production coefficients are constant across time. With2411 observations and two restrictions, the critical χ value is 3.00. In thecase of labor, although the coefficient values po<strong>in</strong>t toward labor shedd<strong>in</strong>g onthe part of sample farms over the period 1995-1999, the <strong>in</strong>teraction terms areneither <strong>in</strong>dividually nor jo<strong>in</strong>tly significant at standard test levels (the teststatistic W = 0.63, with a ρ-value of 0.73). Results suggest that production wasmore sensitive to fertilizer use <strong>in</strong> 1997 than <strong>in</strong> 1995, and less sensitive <strong>in</strong>1999, although the <strong>in</strong>teraction terms are neither <strong>in</strong>dividually nor jo<strong>in</strong>tlysignificant at standard test levels (W = 2.43, ρ = 0.30). In the case of pesticides,the year-specific coefficients are <strong>in</strong>dividually and jo<strong>in</strong>tly significant (W =
SHIVELY and ZELEK10113.14, ρ = 0.001). In terms of overall magnitudes, the results of Model 4suggest a decl<strong>in</strong>e <strong>in</strong> the coefficient on labor <strong>from</strong> 0.23 <strong>in</strong> 1995 to 0.08 <strong>in</strong> 1997and 0.06 <strong>in</strong> 1999; a shift <strong>in</strong> the coefficient on fertilizer <strong>from</strong> 0.13 <strong>in</strong> 1995 to0.21 <strong>in</strong> 1997 and 0.02 <strong>in</strong> 1999; and a shift <strong>in</strong> the coefficient on pesticide <strong>from</strong>0.05 <strong>in</strong> 1995 to 0.02 <strong>in</strong> 1997 and 0.19 <strong>in</strong> 1999. Taken together, these patterns<strong>in</strong>dicate a statistically significant reduction <strong>in</strong> returns to scale for the use ofall variable <strong>in</strong>puts—<strong>from</strong> 0.41 <strong>in</strong> 1995 to 0.31 <strong>in</strong> 1997 and to 0.27 <strong>in</strong> 1999.With n = 411, k = 6, the Wald test statistic of 16.98 (ρ=0.01) <strong>in</strong>dicates that weshould not reject the hypothesis that returns to scale were fall<strong>in</strong>g over thesample period, i.e., a period concomitant with the shift <strong>from</strong> ra<strong>in</strong>fed toirrigated production.Although a structural break <strong>in</strong> the data that was not associated withirrigation cannot be strictly ruled out, our familiarity with the study site, basedon repeated field visits, leads us to attribute observed changes <strong>in</strong> <strong>in</strong>put use tothe <strong>in</strong>troduction of irrigation. In addition, despite the fact that the statisticalevidence <strong>in</strong> support of time-vary<strong>in</strong>g technical coefficients is mixed, wenevertheless observe a strong empirical pattern of <strong>in</strong>put reallocation between1995 and 1999 that we attribute to irrigation. To exam<strong>in</strong>e this pattern <strong>from</strong> adifferent perspective, we use the results <strong>from</strong> Model 3 to derive profitmaximiz<strong>in</strong>g <strong>in</strong>put levels for the sample farms.Substitut<strong>in</strong>g equation (4) <strong>in</strong>to equation (2) and solv<strong>in</strong>g the expectedprofit maximization problem yields a set of three straightforward factordemand equations based on equation (3). These are:*L⎡= ⎢w⎢⎣Lα( pe )1β L + β F + β P −1−βP−βF ⎤−1⎛⎞ ⎛ ⎞⎜βPwL⎟βFwLβ ⎥L⎜⎟(5)⎝ βLwp⎠ ⎝ βLwF⎠ ⎥⎦FP**βFw=β wLβPw=β wLLPLF⎡⎢w⎢⎣⎡⎢w⎢⎣LLα( pe β )Lα( pe β )L−1−1⎛ βPw⎜⎝ βLw⎛ βPw⎜⎝ βLwLPLP⎞⎟⎠⎞⎟⎠−βP−βP⎛ βFw⎜⎝ βLw⎛ βFw⎜⎝ βLwLFLF⎞⎟⎠⎞⎟⎠−βF−βF⎤⎥⎥⎦⎤⎥⎥⎦1β L + β F + β P −11β L + β F + β P −1(6)(7)Us<strong>in</strong>g equations (5)-(7), <strong>in</strong> conjunction with the observed annual dataon <strong>in</strong>put and output prices <strong>in</strong> Table 1, we compute profit-maximiz<strong>in</strong>g <strong>in</strong>putlevels. We assume the dummy variable for tractor use is zero and allow allother dummy variables to vary. We compute a simple average <strong>in</strong>put level