Case studies and single case studies, although sounding similar, are two very different methods which psychologists can use to collect their data. They vary in almost all aspects and are non-compatible to each other, that is that you could not expect to generalise the findings of one onto another.
By their nature case studies lend themselves to qualitative data collection. This is due to the fact that a large amount of information on a variety of mediums can be combined in a case study to get a general picture of the situation (http://eric.ed.gov/ERICWebPortal/search/detailmini.jsp?_nfpb=true&_&ERICExtSearch_SearchValue_0=ED415771&ERICExtSearch_SearchType_0=no&accno=ED415771). In a case study one individual is studied in great detail, with all information gathered relates to or originates from this person. Case studies are great tools for psychologists as the information gained is so extensive that they can use it in virtuously limitless ways. Examples of these methods used to great success include the H.M and Clive wearing case studies regarding memory. They have helped psychologists gain an applied insight into the way people use memory and what can happen when abnormalities occur. Such information would be impossible to gain with other methods which rely on more controlled conditions and the need for multiple participants.
However this benefit of case studies, the fact that such a wealth of information can be gained from one person is also its biggest weakness, a distinct lack of generalisation. It is impossible to generalise a case study back to the population because they are just one individual and the data gained is not derived from means, so it violates many of the assumptions required for quantitative comparisons (http://www.simplypsychology.org/case-study.html). They are also more subjective than other methods of research as they are more susceptible to the influence of the researcher’s emotions and interpretations.
Single case designs work in a different way. They are normally qualitative and involve the use of many participants. Each participant in a single case design acts as their own control meaning that there is reduced chance of the effect being down to individual differences (http://informahealthcare.com/doi/abs/10.3109/02699059209008139). These studies are repeatable as the method can be reapplied to other groups of participants. You can also generalise the single subject design back to the population as multiple means are compared. These studies lend themselves to exploration through statistical tests which make the findings easier to sum up.
It can be seen that these two methods though similar in name are very different once applied. It should also be noted that neither is better than the other as they both work better under different circumstances.
statisticalscrutiny 
I think you meant to say that single case designs are usually quantitative, not qualitative? They usually involve standardised tests which are repeated at different times. Either a few times in the same week, month, or over the course of the participants entire life. The longest single case study ever run is still going I believe, and has followed a single set of participants from birth through their entire lives. Their draw-backs are that they are very expensive, time consuming, and many times participants will drop out at some point during the study making gaps in the results. So are they really worth it? Can we really learn that much more about someone by studying them for their entire life that we couldn’t figure out in a few years?
i think that the quantitative an experiment is, the better and more applicable its results can be. For example, In most cases, case studies are on people suffering mental defects or disabilities, such as Thigpen and Cleckley’s study of Eve White (1950’s) this brought out tons of detailed research and notes of behaviours and feelings, but very little comparable data. i think case studies can be fascinating and very interesting but in my opinion, what they give back to research is very little, or very brief or unable to be applied to others.
Hi statsscrutiny!
When you said this about qualitative research “so it violates many of the assumptions required for quantitative comparisons”. I would say that this might not be a very clever way of framing the sentence because it seems like you are comparing one against each other which is fine, but it seems like you are biding one against the other. As they are distinctively different for a reason, which is that qualitative research is meant for indepth study of an individual or individual’s problem which is what is mostly done in one-on-one counselling setting and even while treating patients in psychiatric units and hospitals. They are all forms of case studies and have equal relevance as quantitative research. Quantitative data is meant to be generalizable whereas qualitative data is meant to be individualistic. Both have there positives and are very context dependent as mentioned in the blog as well. I think you said it in the end but my view would be that like u mentioned that it is the “biggest weakness” of qualitative data that it is not generalizable well, id’ say it’s not meant to be otherwise it wouldn’t be QUALITATIVE. You managed to get in all the pointers of the two researcher styles which is good.