Breaking down the question
The operative word is supplement. The question does not ask you to contrast qualitative and quantitative methods, nor to defend one against the other; it asks how each completes the other in sociological enquiry. The expected answer is therefore integrative rather than adversarial — a case that the two families of method address different weaknesses and, used together, produce a fuller and more trustworthy understanding of social reality than either can alone.
You must first establish what each method is. Quantitative methods — surveys, experiments, statistical analysis — measure, count and seek causal generalisation across large numbers, and belong to the positivist tradition. Qualitative methods — participant observation, in-depth interviews, ethnography, case study — grasp meaning, context and process among small numbers, and belong to the interpretive tradition. Only against that contrast does the idea of supplementation become clear.
At twenty marks the answer should define both, note their respective strengths and limitations, and then show — concretely, with illustrations and the concept of triangulation — the several distinct ways in which they reinforce one another. A balanced conclusion should endorse methodological pluralism.
How to approach it
Begin by characterising the two methods and locating them in the positivist and interpretive traditions, so the reader sees why each has a blind spot the other can cover.
Then set out, as distinct points, the ways they supplement one another: qualitative work generating hypotheses that quantitative work tests; quantitative work establishing patterns that qualitative work explains; each validating the other through triangulation; and each compensating for the other's weakness in reliability or validity.
Illustrate throughout. Durkheim's statistical study of suicide and Weber's interpretive account of the Protestant ethic show the two traditions; a mixed-methods study of poverty or migration — survey plus fieldwork — shows the combination in action. Ground the discussion in the research methods in sociology notes, and close by affirming that the methods are complementary rather than rival.
Model answer
Sociological enquiry draws on two broad families of method. Quantitative methods — social surveys, structured questionnaires, experiments and the statistical analysis of numerical data — aim to measure social phenomena, establish correlations and arrive at causal generalisations across large populations. They belong to the positivist tradition, exemplified by Emile Durkheim's insistence on treating social facts as measurable things. Qualitative methods — participant observation, unstructured and in-depth interviews, ethnography and the case study — aim instead to grasp the meanings, motives and lived experience of social actors in their natural settings among small numbers of people. They belong to the interpretive tradition, associated with Max Weber's emphasis on verstehen, the empathetic understanding of subjective meaning.
Each family has characteristic strengths and matching weaknesses. Quantitative methods offer reliability, breadth, representativeness and the power to generalise, but they can be superficial, missing the meaning behind the numbers and imposing the researcher's categories on respondents. Qualitative methods offer validity, depth, sensitivity to context and access to meaning, but they cover few cases, resist generalisation and are harder to replicate. It is precisely because their strengths and weaknesses are mirror images that the two methods supplement one another. This supplementation takes several forms.
First, sequencing in enquiry — from exploration to testing. Qualitative work is well suited to the exploratory, hypothesis-generating stage. Immersion in a community through fieldwork suggests the concepts and relationships worth investigating; quantitative survey work can then test those hypotheses on a large, representative sample. Exploratory ethnography and confirmatory survey thus form a natural sequence.
Second, pattern and explanation. Quantitative methods are strong at establishing that a pattern exists — that suicide rates vary by religion, or that migration correlates with income — but weak at explaining why. Qualitative methods can then supply the interpretive account of the mechanism. Durkheim's statistics revealed the pattern of suicide; only interpretive enquiry into the meaning of integration and belonging can fully explain the actors' experience behind those rates. The numbers locate the phenomenon; the fieldwork explains it.
Third, triangulation and validation. Using both methods on the same problem allows triangulation — the cross-checking of findings by different means. If a survey and an ethnographic study of the same neighbourhood point to the same conclusion, confidence in that conclusion rises; if they diverge, the discrepancy itself is illuminating and prompts deeper enquiry. Each method thus serves as a check on the other's distortions, improving the overall validity and reliability of the research.
Fourth, illustration and human depth. Quantitative findings can be dry and abstract; qualitative case material gives them a human face, illustrating statistical trends through concrete lives. Conversely, quantitative data can establish how typical a vivid qualitative case actually is, guarding against the danger of generalising from an unrepresentative example.
The complementarity is borne out in practice. William Foote Whyte's Street Corner Society combined close participant observation with attention to the broader social structure of the slum, and modern studies of poverty, migration and health routinely pair large-scale surveys with in-depth interviews and ethnography in a deliberate mixed-methods design. Such designs recognise that social reality has both an objective, measurable aspect and a subjective, meaningful one, and that capturing both requires both instruments.
Assessment. Qualitative and quantitative methods are not rivals forced upon the sociologist as an either/or choice but complementary tools addressing different dimensions of the same reality. Quantitative enquiry supplies breadth, pattern and generalisability; qualitative enquiry supplies depth, meaning and context; and each compensates for the characteristic weakness of the other. Used together — through sequencing, triangulation and mixed-methods design — they yield an account of social life that is at once representative and richly understood. The mature position in sociological methodology is therefore one of pluralism: the choice and combination of methods should be dictated by the nature of the problem, and for most substantial questions, the two families of method are strongest when they work in tandem.
Examiner's perspective
The word supplement sets the task. Answers that merely list the differences between qualitative and quantitative methods, or that argue the superiority of one, misread the question — the examiner wants the complementarity made central and demonstrated.
A solid script defines both methods and locates them in the positivist and interpretive traditions, so that the reasons for their complementarity are visible. The higher band belongs to candidates who spell out the distinct ways the two reinforce one another — exploration then testing, pattern then explanation, triangulation, and mutual compensation for weaknesses in validity and reliability — rather than asserting the idea in general terms.
Concrete illustration lifts the answer: Durkheim and Weber to embody the two traditions, and a mixed-methods study such as Whyte's Street Corner Society or a survey-plus-fieldwork study of poverty to show the combination working. Closing with an endorsement of methodological pluralism, tied to fitness for the problem at hand, secures the top marks.