Re: #9 - From 1973 to 1989 total energy use in this country

[Administrator]

Complete Question Explanation

Must Be True—Numbers and Percentages. The correct answer choice is (C)

Like the vast majority of Must Be True problems, the stimulus does not
contain a conclusion. We are given the following facts, however:

     From 1973 to 1989 total energy use increased less than 10%.

     During this same period, the use of electrical energy grew by more
     than 50%.

     During this same period, the gross national product (GNP) grew by
     more than 50%.

A careful examination of the second sentence reveals that there is no stated
connection between the growth of the GNP and the increase in the use of
electrical energy. If you assume that the use of electrical energy somehow
caused the growth of the GNP, you are guilty of making an unwarranted
causal assumption. Because there is no stated connection between the two
other than they both grew by more than 50%, any answer that attempts
to connect the two is incorrect. Answer choices (D) and (E) can both be
eliminated by this reasoning.

Now that we recognize that the GNP issue is only a red herring, let us
examine the percentages that are given in the stimulus. The 50% increase
in electrical energy gives the impression that the jump must have been
substantial. But we know from Misconception #6 that a large percentage
does not automatically mean a large number. For example, in this
problem it is possible that the 50% increase in electrical energy use was
a jump from 2 units to 3 units. The possibility that electrical energy use
in 1973 was a relatively small percentage of overall energy use directly
undermines answer choice (A), as shown by the following example:

3.PNG

A close analysis of the chart also reveals that answer choice (B) can be
eliminated. In the example, the use of energy other than electrical energy
rose from 90 units to 94 units.

     Although the example disproves answer choices (A) and (B), obviously
     you do not have time to make a chart during the test to examine each
     possibility, so is there a faster way to eliminate the first two answers?
     Yes—consider the previous discussion point that information about
     percentages does not tell us about the numbers. With that idea in mind,
     because the stimulus contains only percentage information (even though
     there are two percentages), you should be very suspicious of answer
     choice (A) (which states that the number of electrical units used was
     greater) and answer choice (B) (which states that the use of non-electrical
     energy declined) since they both contain numerical information. At the
     same time, you should be attracted to an answer such as (C) because it
     contains only percentage information, and as it turns out, answer choice
     (C) is correct.

     Because the misconceptions discussed earlier have a predictable effect
     when you try to make inferences, you can use the following general rules
     for Must Be True questions:

          1. If the stimulus contains percentage or proportion information only,
          avoid answers that contain hard numbers.

          Example Stimulus Sentence:

               The car market share of Company X declined this year.

          Avoid answers which say:

               Company X sold a smaller number of cars this year.

               Company X sold a greater number of cars this year.

          2. If the stimulus contains only numerical information, avoid answers
          that contain percentage or proportion information.

          Example Stimulus Sentence:

               Company Y sold fewer computers this year.

          Avoid answers which say:

               Company Y now has a lower share of the computer market.

               Company Y now possesses a greater proportion of the
               computer market.

3. If the stimulus contains both percentage and numerical
information, any answer choice that contains numbers,
percentages, or both may be true.

Please keep in mind that these rules are very general. You must read the
stimulus closely and carefully to determine exactly what information is
present because the makers of the LSAT are experts at camouflaging or
obscuring important information in order to test your ability to understand
complex argumentation.

[lucastrato]

Hi!

So I seem to consistently struggle with questions like this one. Here's the stimulus and q stem for quick reference:

[Removed by Admin - LSAT questions cannot be posted. Just the practice test number and section, and question number like you posted in the title is all we need!]

How can we be sure that the proportion has shifted? Couldn't it just be that the use of other kinds of energy declined significantly?

Thanks in advance for any help you can provide!

[Dave Killoran]

Hi Lucastrato!

Thanks for the question. This is one of my all-time favorite questions, and if you (or anyone else reading this) has the Logical Reasoning Bible, this question is explained on page 429. The explanation is also in one of our course books (if you are taking a class). Let's take a look at that explanation, because this is a concept you will see again in other LSAT questions.

Like the vast majority of Must Be True problems, the stimulus does not contain a conclusion.

A careful examination of the second sentence reveals that there is no stated connection between the growth of the GNP and the increase in the use of electrical energy. If you assume that the use of electrical energy somehow caused the growth of the GNP, you are guilty of making an unwarranted causal assumption. Because there is no stated connection between the two other than they both grew by more than 50%, any answer that attempts to connect the two is incorrect. Answer choices (D) and (E) can both be eliminated by this reasoning.

Now that we recognize that the GNP issue is only a red herring, let us examine the percentages that are given in the stimulus. The 50% increase in electrical energy gives the impression that the jump must have been substantial. But we know from the general #% misconceptions that a large percentage does not automatically mean a large number. For example, in this problem it is possible that the 50% increase in electrical energy use was a jump from 2 units to 3 units. The possibility that electrical energy use in 1973 was a relatively small percentage of overall energy use directly undermines answer choices (A), as shown by the following example:

1973 1989

Total energy use 100 109
(in units)

Electrical energy use 10 15
(in units)

Percentage of total energy 10% 13+%
use that was electrical

A close analysis of the chart also reveals that answer choice (B) can be eliminated. In the example, the use of energy other than electrical energy rose from 90 units to 94 units.

Although the example disproves answer choices (A) and (B), obviously you do not have time to make a chart during the test to examine each possibility, so is there a faster way to eliminate the first two answers? Yes—consider the point that information about percentages does not tell us about the numbers. With that idea in mind, because the stimulus contains only percentage information (even though there are two percentages), you should be very suspicious of answer choice (A) (which states that the number of electrical units used was greater) and answer choice (B) (which states that the use of non-electrical energy declined) since they both contain numerical information. At the same time, you should be attracted to an answer such as (C) because it contains only percentage information, and as it turns out, answer choice (C) is correct.

In (C), no matter what the exact numbers, if total energy use increased by less than 10% and electrical energy use increased by more than 50%, then electrical energy use had to be a greater overall percentage of the whole (basically, it was growing faster than the rate of the whole, so it became larger relative to the whole).

In response to the point you made at the end, if other types of energy use declined, then the the proportion of the electrical energy in the whole would have to rise even more--there's no way around that because then electrical energy is making up a greater share of the whole (and thus a larger proportion).

Please let me know if that helps. Thanks!

[voodoochild]

Here's how I think about this :

Proportion of elec energy = (1.5) Old Electrical usage / 1.1* total energy usage = greater than 1. Proves C) !

B is tricky because "the other" part could increase too. For instance, 5 = 2 + 3; If 5 increases to 50, it doesn't mean that only 2 will increase, both 2 and 3 could increase!


Hope that helps.

[lucastrato]

Thanks David and voodoochild! I definitely understand how C is the correct answer now.

[curiosity]

I chose B as the correct answer, but after making up some numbers for energy usage, I see that energy other than electrical energy does not necessarily have to decline. However, I'm wondering - since I had narrowed down the answer choices to B and C, what would your strategy be for choosing between B and C when taking the test and you don't necessarily have time to make up numbers and work out the calculation?

[Lucas Moreau]

Hello, curiosity,

I can see how you got stuck between B and C - they seem so similar to each other, don't they? It's a little irritating!

The trick is, we know C must be true, even without working out the numbers. Total energy increased by a little, electrical energy increased by a lot, the same increase - the percentage of total energy that's electrical energy has to have increased.

You don't have the same certainty with B, though! It could be possible that all kinds of energy were used more during that period, just electricity got used a WHOLE LOT more. Or the rest could have remained level while only electricity increased. B is possible, maybe even likely, but it doesn't have to be true the same way C does.

Hope that helps,
Lucas Moreau

[curiosity]

Thank you very much! That makes sense

[VamosRafa19]

I cannot rule out E. I understand from the explanation above that GNP is a red herring. But if total energy went up by 10%, and electrical went up 50% then as C states the proportion of energy increased. However to me this also means in 1973 all other forms of energy combined must have made up a larger % of total energy. Therefore no matter what GNP was, electrical energetic was a smaller proportion of GNP than other forms of energy combined.

[Robert Carroll]

Vamos,

The problem is that GNP and energy use are entirely different things. Electrical energy may have made up 5% of energy use in 1973, but there's no reason it would have to make up 5% of GNP, or even 5% of "that part of GNP dealing specifically with energy use." "Energy use" and "GNP" are entirely different totals with different units of measurement; what is true about electrical's proportion of energy use doesn't need to be true about its proportion of GNP. Electrical may produce a small amount of watts but be really expensive, for instance - it could be a small % of energy use but a huge % of GNP because of that.

If you doubt that reasoning, thinking that maybe the wattage produced is all we're measuring, and electrical produced less wattage in 1973, and thus accounted for less GNP value, note that your reasoning still does not work. Electrical could have been 60 in 1973, all other forms 40; electrical went up to 91, and the total to 109. There is nothing wrong with electrical's already having been a majority of energy use in 1973, and increasing by more than 50%.

Robert Carroll