If I'm understanding your methods correctly, you only have data for earnings by occupation. So when you say "Computer Science majors earn x." it actually means "People who do the same jobs that computer science majors tend to do earn x."
If that's the case then the data is not really actionable unless we also know the probability of getting a job with and without some degree.
For example, if computer science majors all tend to get job A, but having a computer science degree makes you no more likely than people without computer science degrees to get job A, then the value of the computer science degree is 0.
Of course having a degree does usually provide some advantage at getting a job, and in some fields it's impossible to get a job without a degree.
But still, for this data to be truly meaningful and actionable you must multiply the value of each degree by the amount that the degree aids you in getting a job, which is a hard value to determine.
Edit:
Here's an example of why this is important.
Let's compare physical therapy and programming.
According to the data, a Doctorate in Physical Therapy has a value of about $1.2 million, while a Bachelors in Computer Science has a value of around $1.5 million.
You might conclude that the computer science degree is a better value, especially since it costs less than the doctorate.
However, you cannot be a physical therapist without the degree. So the value of the P.T. degree is $1.2m * 100%, or $1.2 million, because without the degree you have a 0% chance of getting the job (and assuming that having a P.T. degree guarantees you a job).
You can be a programmer without a computer science degree. So the value of the degree is going to be $1.5m times some percentage less than 100. In order for the C.S. degree to be more valuable than the P.T. degree you'd have to be 80% more likely to get a job than a programmer without a C.S. degree. (1.5 * .8 = 1.2)
It doesn't seem like people with C.S. degrees are 80% more likely to get jobs. Especially when you factor in the opportunity cost of going to school. That's a lot of time you could spend working on open-source projects, building websites and businesses, etc. which are all things that count more than a degree.
So the value of a C.S. degree could very easily be less than the value of a P.T. degree.
He acknowledged the flaws in the data collection in the blog posts.
Anyway, this particular flaw doesn't quite work itself out the way you describe. If you want to do out the numbers precisely, you'd multiply out the probability that a person enters a particular occupation given their degree by the lifetime earnings of that occupation, and then sum over all possible occupations.
So say (made up numbers) a CS major has an 80% chance of being a computer programmer ($1.5M), a 10% chance of being a quant ($5M), a 5% chance of being a successful entrepreneur ($20M), and a 5% chance of being a housewife ($0). The expected value of the degree would be .8 * 1.5M + .1 * 5M + .05 * 20M + 0.
Then to find the baseline you'd do the same for lifetime earnings and occupation probabilities of someone who didn't go to college. If they had a 10% chance of being a computer programmer, a 30% chance of being a barrista ($500k), a 40% chance of being a construction worker ($800k), and a 20% chance of being a househusband, that'd be .1 * 1.5M + .3 * 500 + .4 * 800 + 0. Multiply out and subtract to find the value of the degree.
Remember that statistics is about making statements about groups, not statements about any particular individual. That illustrates another pitfall of the data: the question a prospective student really wants answered is "how much will the degree be worth to me". There's at least one study out there that suggests the answer is "zero": all the difference in lifetime earnings is due to correlation and not causation. People succeed because of inborn traits like intelligence, perseverance, and the ability to delay gratitude, all of which also result in someone being able to get into an elite college and pick a challenging major. But controlling for that effect opens up a whole other can of worms, one that you could bill your client another few thousand for writing about. ;-)
You're right, I simplified for the example and assumed only one canonical job for each major. However, that doesn't undermine my criticisms. You'd still have to multiply each job by the amount that the degree helps you get that job.
So it's probably pretty hard to become a quant without a degree, so multiply that by some high number like 95%, but it's really easy to become an entrepreneur without a degree, so multiply that by some low number like 20%.
You're also right about the relevant question being "what is a degree worth to me?" That makes prediction even harder because the value of the degree is largely going to depend on the person's opportunity cost of going to school. Consider Mark Zuckerberg, Facebook was the opportunity cost of his getting a degree, which would make the degree's "value" be several billion dollars negative.
I think this data is a lot more accurate and actionable than "All degrees are equally valuable, honey", which is the advice I'm most concerned with competing with.
I have to agree with the parent poster, especially given that I have a degree in International Relations but earn a high 'computer science degree' salary since I work in the field. Anecdotally, quite a few coworkers over the years at different positions have also revealed that their degrees are not in the field, but since, like me, they had the skills in it and the experience, they chose to do that kind of work, despite the degree being in a different subject.
I work for a university, and I don't think I have ever heard a faculty or staff member say "All degrees are equally valuable", a google for such only shows 6 results. All with the word not in front. Even doing more liberal searches produces the same results, pretty much every single article says no. Where is the advice you are supposedly getting? Is it coming from an authoratative source? You could really use a citation.
Yeah, sure it is, because that's horrible advice. Does anyone actually believe that all degrees are equally valuable?
I think people have a general idea of which degrees are more valuable than others, a flawed and vague idea to be sure, but it's not obvious that this data is more accurate than that.
I wonder if you couldn't get very similar results by asking people on Mechanical Turk to rank degrees by what they thought was their value.
Regardless of how it fares against other advice, it still has flaws that can significantly affect its accuracy; and I'd want anyone I cared about to understand that before making decisions based on it.
If that's the case then the data is not really actionable unless we also know the probability of getting a job with and without some degree.
For example, if computer science majors all tend to get job A, but having a computer science degree makes you no more likely than people without computer science degrees to get job A, then the value of the computer science degree is 0.
Of course having a degree does usually provide some advantage at getting a job, and in some fields it's impossible to get a job without a degree.
But still, for this data to be truly meaningful and actionable you must multiply the value of each degree by the amount that the degree aids you in getting a job, which is a hard value to determine.
Edit:
Here's an example of why this is important. Let's compare physical therapy and programming.
According to the data, a Doctorate in Physical Therapy has a value of about $1.2 million, while a Bachelors in Computer Science has a value of around $1.5 million.
You might conclude that the computer science degree is a better value, especially since it costs less than the doctorate.
However, you cannot be a physical therapist without the degree. So the value of the P.T. degree is $1.2m * 100%, or $1.2 million, because without the degree you have a 0% chance of getting the job (and assuming that having a P.T. degree guarantees you a job).
You can be a programmer without a computer science degree. So the value of the degree is going to be $1.5m times some percentage less than 100. In order for the C.S. degree to be more valuable than the P.T. degree you'd have to be 80% more likely to get a job than a programmer without a C.S. degree. (1.5 * .8 = 1.2)
It doesn't seem like people with C.S. degrees are 80% more likely to get jobs. Especially when you factor in the opportunity cost of going to school. That's a lot of time you could spend working on open-source projects, building websites and businesses, etc. which are all things that count more than a degree.
So the value of a C.S. degree could very easily be less than the value of a P.T. degree.