Reading a Rainfall and Maize Graph
A learner is given a bar-and-line graph for a village: bars show annual rainfall in millimetres (1999: 610; 2000: 480; 2001: 720; 2002: 350; 2003: 520; 2004: 300; 2005: 640), and a line shows maize harvest in tonnes (1999: 90; 2000: 70; 2001: 105; 2002: 40; 2003: 75; 2004: 35; 2005: 95). The learner writes: 'Harvest goes up whenever rainfall goes up, so rainfall alone decides the harvest. 2004 proves it - least rain, least maize.' A teacher notes that fertiliser prices doubled in 2002 and stayed high, and that the 2003 harvest came after a government seed subsidy.
1. Test the claim that rainfall alone decides the harvest, using at least two years as evidence.
Read the worked answer
Correlation is strong but not perfect. Ranking years by rainfall: 2004 (300 mm) lowest, 2002 (350) next, 2000 (480), 2003 (520), 1999 (610), 2005 (640), 2001 (720). Ranking by harvest: 2004 (35 t) lowest, 2002 (40), 2000 (70), 2003 (75), 1999 (90), 2005 (95), 2001 (105). The orders match, so rainfall is clearly a major driver - the learner is right about the general pattern.
But two years break the 'alone' claim. 2003 had 520 mm, close to 1999's 610 mm, yet produced 75 t against 90 t - lower harvest on only slightly lower rain, in the year after the seed subsidy. And 2000 (480 mm) gave 70 t while 2003 (520 mm) gave 75 t, a small gain for more rain. If rainfall alone decided the harvest, the ratio of tonnes to millimetres should be roughly constant: it is 0.148 in 1999, 0.146 in 2000, 0.146 in 2001, 0.114 in 2002, 0.144 in 2003, 0.117 in 2004, 0.148 in 2005. The two low ratios are 2002 and 2004 - the years after fertiliser prices doubled.
So the evidence supports 'rainfall is the main driver' but not 'rainfall alone'. Input costs and subsidies also move the harvest.
2. Explain how the fertiliser price rise and the seed subsidy change the interpretation, and state one limit of the graph as evidence.
Read the worked answer
If fertiliser doubled in 2002 and stayed high, farmers in 2002 and 2004 may have applied less, lowering yield for the same rain. That fits the low tonnes-per-millimetre ratios in exactly those years (0.114 and 0.117 against about 0.146 elsewhere). The 2003 subsidy would then explain why 2003 recovered to 0.144 despite modest rain. On this reading the graph shows rainfall setting the ceiling and input costs deciding how close farmers get to it.
Limit of the graph. It has only seven years and one village, so a single bad year can create an apparent rule. It also gives no information on how much fertiliser was used, on plot sizes, on pests, or on whether the maize was sold or eaten - so it cannot by itself prove causation. The teacher's note is outside the graph; the graph alone shows association, and the extra information is what turns association into a plausible explanation. A stronger study would compare villages with and without the subsidy in the same rainfall year.