DI Step Method
5.1 DI X-Ray
 
5.2 DI Fundamentals
 
 
5.3 DI Step Method
 
5.4 Practice Problems
 
DI Step Method
You’ve now seen numerous DI scenarios and worked your way to many DI answers (and even one nonanswer). It’s time to put your knowledge to work in the context of our DI step method. Here’s a rundown of the steps:
Step 1: Get the Specs.
Step 2: Grill the Interrogator.
Step 3: Gather the Data.
Step 4 (if necessary): Power Through.
Why is Step 4 labeled “if necessary”? What does “Grill the Interrogator” mean? How does the little refrigerator light turn off when you close the door? All excellent questions! All (except that last one) will be answered right now as we take a closer look at each step.
Step 1: Get the Specs. You’re familiar with this step from both the Problem Solving and Quantitative Comparison step methods. The difference here in DI is that you have to scope out and get a firm handle on the elements of the diagram(s) before you have any chance of answering the questions. So, the “specs” in this question type are the parameters of the graphs or tables presented. You need to perform this step only before the first question, since the diagrams don’t change from question to question. Issues you need to notice up front include the following:
  • What does the data represent?
  • What is the scale of the data presented? Make sure you know how the things represented in the diagrams are measured. Take note of the units (thousands, millions, feet, seconds, hours, etc.). If a diagram contains multiple scales, notice whether they’re the same or different. In tricky DI sets, multiple scales may be indicated in different units, which makes eyeballing much more risky.
  • If there is more than one diagram, what’s the relationship between them? Does any obvious correlation emerge? Does one diagram contain information that represents a subset of the data contained in the other diagram?
  • Are any trends evident from eyeballing the data? Take a quick look at the big picture to help you get the gist of the information before moving on to the first question.
Step 2: Grill the Interrogator. Once you know what the diagrams are about, it’s time to tackle the first question. Each question provides a wealth of information that should alert you to the math concepts (if any) in play and direct you to the data you’ll need to answer it. Grill the question itself to unearth clues that will lead you in the right direction. When you know what they’re asking, you’ll recognize what you need to know, which will further help you determine where you need to look.
Step 3: Gather the Data. By this point, you’ve gotten the diagrams under your belt and have determined what the question is asking, which should allow you to venture forth to find the data you need to answer the question. In some cases you may also need a Math 101 concept to set you on the right track, but you can get by on many DI questions without “mining the math” (Step 3 of the PS and QC step methods). You certainly should know how to work with percentages and be familiar with common data analysis concepts, but overall you shouldn’t have to dive too deep into your Math 101 knowledge base. If the question set contains only one diagram, use your information from steps 1 and 2 to direct you to the part of that diagram containing the data relevant to the question at hand. If the set contains multiple diagrams, then use the information in the question to tell you where to head, in what order, to get the numbers you need.
Now, it’s possible you won’t need to “Power Through” in Step 4, which is why we label this final step if necessary. That’s because the simpler DI questions test only whether you understand the question asked and can read the answer right off a graph or table. In those cases, you’ll be done by Step 3. Questions answered by eyeballing generally fall into this category. More complex questions, however, require not only that you find the relevant information but also that you manipulate it after you have it. That’s where Step 4 comes into play.
Step 4 (if necessary): Power Through. If you can’t simply read the answer off a graph or table, you’ll have to crunch the numbers you extracted from the diagram(s) in Step 3. You may need to calculate precisely, or you may get away with approximating. In any case, expect many DI questions to require at least a bit of work after you’ve gathered your information.
Guided Practice
Don’t worry if you’re not totally on top of DI just yet. It’s going to take practice, more of which awaits you in the practice set at the end of this chapter and the practice test at the end of the book. But first we’ll walk you through our step method in the context of another question, resurrecting one last time our good old home-building scenario.

Price per Square Foot of Exterior Surface Materials from 1940 to 1990

1940 1950 1960 1970 1980 1990
Siding

$4.59

$4.79

$5.79

$6.29

$8.99

$8.99

Brick

$12.00

$12.00

$15.00

$16.50

$22.50

$34.00

Wood

$9.00

$11.50

$11.00

$13.00

$15.00

$18.00

If new homes built with siding in 1970 averaged 915 square feet of exterior surface material, approximately how much money was spent on siding for new homes in that year?
(A) $700,000
(B) $3,600,000
(C) $29,000,000
(D) $46,000,000
(E) $140,000,000
Step 1: Get the Specs. We’ve seen these diagrams already, but let’s reiterate the specs for good measure. There are two diagrams, one graph and one table, each containing siding, brick, and wood information for six different years. The multipart graph depicts the number of homes built with each surface material, and the scale is in thousands. We need to be careful to consider the correct beginning and end points for each segment of the graph. The note at the bottom eliminates the possibility of multiple-surface houses, averting possible confusion on that front. The table contains the prices per square foot of the material in the various years. Prices generally seem to rise across the years, with one exception in the wood row.
Step 2: Grill the Interrogator. The question limits its inquiry to 1970 siding homes, which narrows things down considerably. It provides the average number of square feet per siding homes built that year and asks for the approximate amount of money spent on that surface in 1970. The word approximately and the large difference between the dollar figures in the choices tell us we can estimate the values to make our lives easier. That’s a great jump on the question, so let’s move on to Step 3.
Step 3: Gather the Data. We know that money is involved, so the table will come into play, but what else do we need to know to calculate the total amount of money spent on siding in 1970? The basic formula total cost = cost per unit × number of units comes into play. In this case, number of units is itself broken down into average number of square feet per house (915, given) and the number of houses built in that year. So the total siding cost in 1970 = number of siding houses built in 1970 × 915 square feet per house × cost per square foot of siding in 1970. The cost per square foot of siding in 1970 is $6.29, and the number of siding homes built in 1970 was around 8,000. (The siding segment in that year starts just around 60,000 and ends before the 70,000 line.)
Step 4: Power Through. We’ve got our figures, so let’s bring it on home: Since we’re looking for an approximate value, we can round 915 square feet per house up to 1,000 to make matters simpler. Multiplying that by 8,000 houses gives us 8,000,000 square feet of siding used for new homes in 1970. While we’re at it, let’s approximate $6.29 as plain old $6.00, which gives us 8,000,000 square feet × $6.00 per square foot = $48,000,000. Is there anything close among the choices? Yup—$46,000,000, choice D. That number makes sense too, since we rounded 915 up to 1,000, so we should expect that our answer might skew a bit high, despite the fact that we also rounded the price down a bit.
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