Overall problem: relationship to 'time'
i) longitudinal (following the same learners from Time 1 to Time N)
ii) cross-sectional (taking a sample usually of dfifferent people at Time 1, Time 2, Time N and comparing)
iii) experimental time (i.e. ultra short-term 'learning' in experiments etc)

General methods
the observational method - language elicitation and analysis e.g. Hatch
the difference method - measurement of learner or situational variables and correlation with proficiency or something else
the manipulative method - treat one group, not the other, measure effects of treatment

Types of evidence
i) introspection: thinking aloud, self-observation, grammaticality  
    judgements
ii) natural data; e.g. Faerch, etc
iii) controlled data

1. Is speech more important than writing?

2. Is speech more important than writing in L2 learning?

3. Is speech more important than writing in the L2 learning of
    vocabulary?

4. Is speech more important than writing in the L2 learning of English
    vocabulary by French adult learners?

5. Is speech more important than writing in the L2 learning of English
    vocabulary by French adult learners being taught in technical schools
    in France?

6. Is a group of French adult learners of English in technical schools in
    France which is taught orally better at learning vocabulary than an
    otherwise identical group that is taught through writing?

7. Are the scores on the EPVT of a group of French adult learners of
   English in technical schools in France who are taught orally better than
   an otherwise identical group that are taught through writing?

8. Are the scores on the EPVT of a group of French adult learners of
   English in technical schools in France who are taught orally
   significantly better than an otherwise identical group that are taught
   through writing?

Amplification of this sequence will be found in Chapter 1 of V.J. Cook (ed.), Experimental Approaches to L2 Acquisition, Pergamon, 1986.