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An Interview with Watts Humphrey, Part 21: The Personal Software Process

In this transcript of an oral history, Grady Booch interviews SEI Fellow Watts Humphrey. In Part 21, Humphrey discusses the creation of his Personal Software Process (PSP).

This interview was provided courtesy of the Computer History Museum.

See the entire interview.

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The PSP Research

Humphrey: I decided that what I would do would be to try applying the CMM principles to my personal work,and so I did. I tried to use it on balancing checkbooks, putting together simple processes, and that didn’t tell me anything. So I decided I’d better write some programs. I hadn’t programmed in years, but I was still running the process program at the SEI, and I decided I would start writing Pascal programs because I hadn’t written anything in a long time. And so I started doing that, and I was asking the guys at work for help and then getting some guidance because they were a lot more recent programmers than I was.

And so that’s how I started the PSP [Personal Software Process] and it was extraordinary. I wrote a few programs. I had just gotten started and Larry Druffel called me in one day -- he was thinking of making me a fellow -- and I said, “Well, what would that mean?” I had retired from IBM. My mother was living in Sarasota, Florida with my stepfather and -- I think I told you my mother and father had been divorced. Well, my mother had married a gentleman, Allen Osborne, a marvelous gentleman, really a prince of a guy -- and they had come to live in Florida. It turned out Allen Osborne was an old friend of the family and he had been a pilot in World War I and he was my father’s flight instructor in World War I. Taught him to be a fighter pilot. My dad must have been a wild man because they called him the German Ace; he cracked up five American planes.

So it was an amazing family connection. But in any event, they were living in Sarasota, Florida so when I retired we came down here. My mother died within about eight months of when I retired, and we came down for the funeral and decided we’d buy a house. So we bought a little house and we used it as a vacation home. We bought that in ’86 and this is about, oh, three years later, ’89. I’m managing the SEI Software Process Program, my book had come out, and it was wintertime and snowy and not very pleasant in Pittsburgh. Barbara said, “I really would like to live in our house in Florida.” It seemed to me that was fair, but I didn’t know how I could do that with what I was doing.

So I went to talk to Larry Druffel about it and he said, “Well, we could make you a Fellow.” And I said, “What’s that mean?” He said, “That means you can work on what you want to work on where you want to work.” And I said, “And you’ll pay me, too?” And he said, “Oh yeah,” and I said, “That sounds pretty good to me.” So I agreed and he set up a whole Fellow structure and everything else, and it went through a procedure and a committee and they made me a Fellow. And so I got myself replaced. I got Bill Curtis to come take my process job, and I decided to come down here and work on the PSP. The CMM was going already. I wasn’t really that deeply involved. I was concerned with it because it wasn’t really nailed down and I wanted to make sure it didn’t go off course, but they were doing their own thing. I got busy doing the PSP, so that’s what I did.

Booch: Back to Bill Curtis, it’s interesting that his name came up here because he had been involved in -- wasn’t there some semiconductor consortium down in Texas around that time from where he came?

Humphrey: Yes, the MCC.

Booch: The MCC, yes, that’s where he came from.

Humphrey: He was down there. He worked for Les Belotti.

Booch: Yes, Les as well. All these names come back. Right.

Humphrey: He worked for Les Belotti down there in the MCC. I remember going to a meeting with him and visiting with him, and they had quite an interesting little group down there. They were doing great stuff, but they were having a real problem getting industry support -- particularly for the software -- and that stuff essentially wasn’t getting anywhere. And so Bill did come and replace me. I think it was about January ’90 I finally quit that job and came down here and, as I say, I wrote a total of something like 72 programs in Pascal, and then I moved to Object Pascal, and then I moved to C++.

They were all, kind of, statistical programs and, sort of, analysis programs -- stuff that wasn’t particularly useful for anybody -- but it was stuff that I was interested in and measuring things and stuff like that. So, kind of, programs I made up with little requirements, but I had a whole lot of practices I was trying to follow, and I pushed reuse and as a matter of fact, I made reuse libraries when I got into Object Pascal and then C++. So I built these big class libraries and I had all this reusable code. I got to where I was writing programs where something like 80% of the code was reused. It was enormously effective, and the thing that was most effective about it to me was several things.

I put in some practices for how I do it, but the quality results were extraordinary because I was using the PSP. I was measuring quality. I was managing it and by and large my programs would have zero or one or two defects in compiling and testing. I’d have practically no defects and the programs ran. I mean, I basically never had problems. My reusable programs, I’d just turn them on and I’d plug them in and they would work and it would be great.

So I could build and compose these great big systems without any real trouble, and I had a series of procedures for how you write reusable programs. And basically what I did, I had a user’s manual as the opening of the listing. I had instructions on how do you use the program, what are the calls and returns, what are they, and then there’s a series of warnings about things you can’t do and shouldn’t do, but that was basically it. And so these programs I discovered -- it was a dream. I was able to compose programs and stuff.

When I wrote the very first programs I decided that I was going to measure everything because I didn’t know what to measure, so I’ll measure everything. And so I decided to measure how much time it took me to do everything, how many defects I found and fixed at each step, how big everything was that I built, and it turned out that’s all I had to do. I measured that and that’s all the measures I needed, that’s all I could get basically. That’s what you do. First thing I discovered the minute I wanted to measure, though, I had to have a defined process because you can’t define a measure without having a process. What’s a defect? You got to know when you found it and when is defined by your process.

So basically when I wrote the first program, I was going to sit down and write it and I realized I couldn’t do this until I had a process, so I decided to put together a process. As I got developing the process for my very first program, I realized I’d made my first mistake. So I backed up again. So I next developed a process for developing a process. So that’s what I did first. I put together a very simple development process, and then I used that to develop my software process, and then I used that to develop my first program. If you remember, I told you that the way I learned to write programs was when I was inventing instruction sets. So I would actually go back and change the instruction sets when I was writing programs early on.

Well, I couldn’t do that with Pascal, but I could do it with a process. So I would write following the process, and then I realized that when I was finished with the process that I really didn’t like the way the process worked in these ways. I put in a little procedure for myself. It’s called a PIP, Process Improvement Proposal, and so I made a note as I was writing my programs of the things that were inconvenient about the process, and I tuned my process, and I kept doing that. I had a problem because if I tuned it too much, of course, my measurement system was screwed up because I couldn’t measure and track from program to program.

So I had a problem with compatibility there, and I had various versions I went through. And so I gathered data. I had data on every defect. I’ve got it all here. I’ve got all these programs. It was all done by hand. I didn’t have any tools to support me. I just had the regular compiler. So I gathered all this data by hand which was a real pain, but I found it didn’t take very long. I was quite surprised how little time it takes to track your time and defects and measure size of a program. I mean, it couldn’t have been more than 5% or 10% of my time, if that.

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