Chris Dillon has an interesting article: Bad Arguments against Marxism. Quite aside from the actual content, Dillon says, "You might object here that my Marxism is idiosyncratic. Certainly, it owes more to the Marx described by Jon Elster than to the one portrayed by Leszek Kolakowski. But frankly my dear, I don’t give a damn." But Dillon misses a crucial point: everyone's Marxism is in some sense idiosyncratic.
The problem with "Marxism" (and similar terms such as socialist or communist) is that there really isn't anything that actually "is" Marxism. Marxism is a genetic or self-identifying label: if your work or ideas derive from Marx's, then you're a Marxist; if you call yourself a Marxist, then you are a Marxist. Pretty much every mainstream sociologist and anthropologist is a Marxist. There are some restrictions: if you want to call yourself a Marxist, and almost all other people who call themselves Marxists don't buy it, then the label probably won't stick. But people who call themselves Marxists (which I myself do only with great reluctance) are a pretty pluralistic group.
In a sense, calling yourself a Marxist is like calling yourself an American: I call myself an American because I was born here, I've lived here all my life, and my beliefs and preferences were shaped in no small part by the predominant culture. But the label does not constrain my beliefs: I can believe literally anything at all (save that I don't want to call myself an American) and I would still be an American. The label refers to nothing ideologically essential.
One could create statistical measures of what Marxists believe, as one could create statistical measures of what Americans believe, but those measures would not define Marxism: they would be posterior measures, not prior definitions.
First, it is more useful to talk about socialism rather than Marxism for the same reason that nobody calls physicists "Newtonists," and evolutionary biologists don't like being called "Darwinists": the label encourages an undue reliance on the work of a single author, however seminal. Although they offered groundbreaking insights that are still in use today, Newton was mistaken on many points, Darwin was mistaken, and similarly, Marx was mistaken (or some now disagree with him). But so what? Newton, Darwin, and Marx were not prophets. Moreover, people who follow in Newton's, Darwin's, and Marx's tradition, who employ their insights, disagree about many important points. This is as it should be.
Second, it is not just pointless but pernicious to talk about socialism as some political-economic system with a particular essential (but arbitrarily designated) definition. To do so creates a false dichotomy: all we have is capitalism and socialism (if you're neither capitalist nor socialist, you're nothing at all), and socialism is bad because reasons, therefore capitalism. It's a move designed to defend capitalism without actually defending capitalism.
It is more useful to talk about socialism not as some specific system but as a project. Socialism is first a project to engage in radical criticism of capitalism, i.e. to carefully and ruthlessly (i.e. without sentimentality or fear) examine capitalism at its roots, at the private ownership of the means of production. Socialism is second a project to imagine a society that functions for the benefit of the people, all the people, and not for the benefit of some elite. The rest is commentary.
But, oh! what commentary! Just within the above constraints, there is vast disagreement. Do we impose socialism from above with a vanguard party seizing state power? Do we let it "bubble up" from below? Is socialism inevitable — all we have to do is wait, and it will spontaneously emerge — or does socialism require definite, intentional action? Given that capitalists will defend capitalism to the death, how far should we go to oppose it?
How do we deal with macroeconomic issues in a national and global economy? Should we industrialize further or deindustrialize? What importance should we place on specifically material well-being? Are washing machines, air-conditioners, computers, synthetic insulin, and (relatively) painless dentistry worth having in the first place? Should all live in small self-sufficient rural communes?
How do we deal with discrimination based on race, sex, sexual orientation, gender identity, national origin, and religion discrimination? How about religious extremism? How do we deal with ordinary crimes, such as rape and murder?
All we can do is engage in the social process of criticism and argumentation to wrap our heads around all these issues. People will disagree, dominant ideas will change, and if we finally do end capitalism, at least our grandchildren will have a rich, albeit internally contradictory, body of thought to employ in solving concrete problems.