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There's a lot in this brief question, and I would like to try to give a brief answer, so I'm going to pick and choose what I respond to (and others might choose different things). Here are the parts of this question I see:

Is teaching multiplication as repeated addition problematic? Is the problem with teaching multiplication as repeated addition that it makes it difficult to differentiate the two operations (or see them as independent)? Is multiplication too hard to teach without using repeated addition? Is multiplication as repeated addition more intuitive? Is there research on teaching methods for multiplication that do not use repeated addition?

Not all these are questions you are asking directly, but they are implicit (such as when your friends assert that repeated addition is more intuitive -- I have problematized it).

We could also well ask "Is multiplication repeated addition?"

Because if we're going to wonder whether we should teach multiplication as repeated addition, we should probably at least consider whether it is repeated addition. If it is repeated addition, then this isn't a problem to teach it that way. If it isn't repeated addition, then why would we teach it as repeated addition? Now, repeated addition may be a strategy students use in certain situations. But that's different from teaching multiplication as repeated addition.

OK, I'm going to draw heavily from Simon and Blume (1994) because it's such an interesting paper for many reasons and it references other resources that address multiplicative reasoning, and this will allow me to be a little lazy. The article is actually about elementary teachers understanding of area as a product of linear measures.

In this paper, Simon and Blume (1994) on page 474 reference earlier works by Kaput and by Schwartz which points out that multiplicative reasoning can result in the production of intensive quantities (that is, a quantity that is not counted or measured directly and is invariant with the scale of the system). This has implications in understanding proportion later. We see intensive quantities in division. Miles traveled divided by hours elapsed produces an entirely new quantity (speed).

The paper suggests "intensive" vs. "extensive" quantities may help us understand not just whether a student is solving a multiplication problem, but how sophisticated their approach is. This idea comes from Thompson (1994) who observed students solving problems relating to speed without conceiving of speed as an extrinsic quantity, revealing something about the sophistication of their conceptual understanding.

All that is simply to point out that there is some complexity to how learners may think about multiplication, and it is worth paying attention to not only because it is part of how multiplication makes sense.

The property of multiplicative reasoning to produce a quantity that is different from the factors in the problem can be considered a referent-shifting aspect of multiplication. Simon and Blume explain:

Schwartz argues that multiplication and division are "referent transforming" operations, because they take two quantities with different referents as input and output a third quantity whose referent is different from either of the first two (in Example 1: number of cookies x number of dollars/cookie = number of dollars). Schwartz and Kaput further point out that the notion of multiplication as repeated addition is problematic because addition is referent preserving, whereas multiplication is referent transforming. Repetitions of addition therefore cannot yield the referent that is appropriate for the product in a multiplicative situation.

So, yes to 1 -- there is reason to believe that repeated addition is a problematic way to teach multiplication: it may obscure the referent-transforming aspect of multiplication. To number 2, you may want to investigate the idea of independence further, but clearly there are other reasons that repeated addition has a different meaning than multiplication. In some sense: yes; failing to make clear the meaning of multiplication is a lost opportunity to separate the operations from one another.

I'll address #4 in a limited way by saying that we don't necessarily want to reinforce student intuitions. So the question of whether it is more intuitive or not may be moot. Is it helpful to our students in achieving the ability to reason multiplicatively?

I will address #3 and #5 together by directing you to a couple of other resources, but also by giving you an example.

Kouba and Franklin (1995) give a brief overview of their view of the research on introducing students to multiplication and division. Their conclusion is that students need a varied conceptual basis for multiplication and division. They include an example of students using objects they can touch in the process of scalar multiplication (which they may accomplish by using repeated addition as one of their own strategies).

However, as a teacher, how can we help students conceptualize multiplication in a way that is consistent with Kaput's and Schwartz's observation that multiplication involves a referent transformation? I look to an example in Mathematics for elementary teachers (Beckmann, 2010).

Dr. Beckmann gives a number of different examples for modeling multiplication, but in one example she uses an array of soft drink cans to show how the idea of cans-in-groups and then number of groups can be put in a useful representational structure. She points out that the rows or the columns can be used as the groups. I would also add that this lets us see that this is not just repeated addition of cans; this representation really does show referent transformation: cans per row * rows = cans also cans per column * columns = cans. This is conceptually different from saying "what's three times five cans?" One obvious difference is that the numbers have meaning in the array model, when we talk about them as a number of groups, or a size of a group.

And, if you want to discuss repeated addition, this model allows us to show why, in this case, repeated addition gives us the correct answer for the multiplication problem.

Repeated addition can be something you do, but it can be separate from a conception of what multiplication is.

In summary

There is reason to question the teaching of multiplication as repeated addition on the basis of the other meanings and understandings of multiplication we want for our students. This is discussed in some of the research that highlight the differences between the reasoning that repeated addition produces and multiplicative reasoning. Students need a varied conceptual basis to form an understanding of multiplication; examples of these can be found in the resources cited, along with representations that support them. Student use of repeated addition is one strategy. I gave an example of how a representation could possibly be used to connect this strategy to another conceptual basis for multiplication.

Cited:

Beckmann, S. (2010). Mathematics for elementary teachers. New York: Pearson Addison-Wesley.

Kouba, V. L., & Franklin, K. (1995). Research into Practice: Multiplication and Division: Sense Making and Meaning. Teaching Children Mathematics, 1(9), 574–77.

Simon, M. A., & Blume, G. W. (1994). Building and understanding multiplicative relationships: A study of prospective elementary teachers. Journal for Research in Mathematics Education, 472–494.

Thompson, P. W. (1994). The development of the concept of speed and its relationship to concepts of rate. The Development of Multiplicative Reasoning in the Learning of Mathematics, 179–234.