If you want to see whether medical statistics/epidemiology appeals to you, you could always try this free online course:
https://www.edx.org/courses/HarvardX/PH207x/2012_Fall/about

Agreed. The old 'attack the issue, not the person' rule comes in handy. In your original wording, it comes across that you are criticising the author's shortcomings in having failed to do something. Consider instead:
"Though the study by X covered points Y and Z, the absence of empirical data weakens the argument." etc
This then gives you the scope to present your own work as building upon prior work and plugging a gap, rather than coming across as though previous work has been a bit shoddy (even if it has!)

I know the passive voice gets a kicking sometimes, but it can be handy for diplomacy!

Is this for a paper or for your thesis? If the latter, the reality is that he will probably never read it.

It does feel a bit awkward (I've also obtained papers from authors, which I've turned out to disagree with) but critical appraisal of the literature is important, regardless of how nice people are. Just be really really sure you're right!

A further point to add - when I first thought about quitting, I wondered whether some of the issues were down to me not giving the process a fair chance/not making enough effort. So I really threw myself back into my project, and really genuinely tried to make it work, but several months on everything was still the same and I decided, 'Enough is enough'. So it wasn't an overnight decision.

I quit a PhD once and I have no regrets. I'm now halfway through another and very happy with it.

In my case, there were several reasons for quitting. I had developed an interest in a different field which I had already decided I would move into in the future. My project wasn't going brilliantly and I didn't have much conviction in the direction in which I was being steered. I didn't enjoy the day to day tasks required for that particular project. And the environment I was in was, shall we say, less than supportive. So in the end I decided to cut my losses and leave, rather than battling through to the end of something that was making me miserable. Before doing so I sought advice from someone in the field that I wanted to move into, which ended up leading to a job, then a masters and now a PhD. I knew research was right for me, and I was right for research, but my first attempt was a case of wrong place, wrong time.

I think the important thing is to pinpoint why you want to quit and what you would do instead. It may be that whatever is making you think you want to quit is in fact fixable. If you quit, then would it be a barrier to whatever career you choose to pursue? In my case it hasn't held me back as people seem to view it on my CV as general research experience that has generic value - but if I'd wanted to stay in exactly the same field, I can see it might have been perceived as more of a problem. If you want to leave research/academia entirely, then you need to make sure that you can identify transferable skills that you can sell in job applications. Outside of academia, PhDs can be viewed with suspicion, so I'm not sure how half-finished PhDs are considered.

All jobs have their stresses - don't fall into the 'grass is greener' trap. As Dalmation says, stress is common, as there is a lot of learning to do. This can be positively channelled though - I re-read this essay on 'productive stupidity' frequently for reassurance! http://jcs.biologists.org/content/121/11/1771.full Depression is also not uncommon among PhD students, though one should be careful about inferring that the PhD is the cause. It need not be a barrier to carrying out a postgraduate study, but if the PhD is a trigger then obviously ways of handling that need to be found.

Ultimately it's a decision that you need to make for yourself, and with a good understanding of what your true reasons are. For me it felt like an enormous decision to make at the time, but a few years on (and having recovered from the damage done during my first attempt) it feels more like a blip in my career than a major life changing event.

Good luck with the thinking process.

Just re-read that - if you're getting the tube, yes, probably get an Oystercard. Single tube fare is about £4 without one.

Oyster card will cost you a fiver but worth it if you're likely to visit London again. If this is just a one off trip though, possibly not.

Whether you need one on this occasion really depends on where you're starting from! If you're just making two single bus journeys you might not have much to gain. Again, the TFL website will have advice on fares etc.

Be aware that some bus journeys in central London require you to buy a ticket from a machine before boarding, so do have some change.

Or maybe N9?

Maccle is right - the TFL journey planner website is extremely helpful on such occasions. For journeys that might be ok to take on foot, I'd also recommend Walk It: http://walkit.com/

It might be worth considering a stint as a research technician/assistant first in that case, to see if your expectations of research match reality. The new developments are rewarding, but there's often a lot of not so exciting slog in between. My undergrad summer and dissertation projects were a good intro to research, but didn't really prepare me for the day to day realities of the long haul.

Getting some experience would also help you when it comes to putting in PhD applications. Getting to know a research group before starting a PhD can be advantageous, particularly if trying to decide whether someone would make a good supervisor.
As Alleycat says, an MRes would be another thing to consider, as a way of bridging the gap and getting your foot back in the door.

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Denying all knowledge is a terrible idea - if your friend made a genuine mistake then denial of it will make him/her look guilty.

It's almost certainly the case that the thesis will have been run through plagiarism checking softwarre, though as others have said, it might be too minor a case to pick up. I used to copy edit for a student academic magazine, and I could spot a mile off when people had pasted in text from elsewhere, cos the tone/style would change. Hopefully a PhD candidate will write well enough by this stage that such an error wouldn't be glaringly obvious, but I wouldn't assume the mistake would go unnoticed just because it wasn't detected by software.

If it was me I would almost be inclined to take pre-emptive action and speak to a graduate tutor to highlight the issue as a genuine error. In a long thesis, accidental plagiarism of a few lines would, I imagine, hardly be grounds to penalise a student, particularly if it's a genuine mistake.

What's your motivation for doing a PhD? Do you want a career in research?

I would say stay on until you've got some publications drafted at least. That is what will be likely to make the difference between you getting an academic job or not. That doesn't necessarily mean staying till the very end of the three years, but you might as well make use of some of the time, particularly as there will be lulls while you wait for feedback on thesis drafts.

As far as the maternity situation goes...I speak from a position of zero personal experience, but I wouldn't make my plans around that as such. It might take a while to get pregnant and you might find it happens when it's too late to actually incorporate maternity leave into your funding period. It can be awkward if leave is taken at/near the end of a position - if you're nearly finished your PhD then what would you be returning for?

You also might like to check the university's/funding body's regulations with regard to finishing early. If you're clearly going to be done and dusted in the next six months, they might not be happy to let you hang around, whether for the purpose of publishing, or to subsidise your maternity leave.

Finally, on the 'originality' front - we were told at my uni that as long as our work was 'original' on the day we started then it was fine as far as the thesis goes. Obviously, the journal world is different, which is another argument in favour of getting publications out sooner rather than later.

It might be worth checking what your university's thesis writing guidelines are and whether they specify a particular format. If they have a preference then you might as well get into the habit now.