The study uses a comparative historical case-study design. The sample is purposive because the objective is to test a known cultural mechanism, not to estimate the percentage of all translated songs that became hits.
The corpus contains 24 cases:
3 cases with an especially strong chronological or documented connection between adaptation and a new market stage;
13 supporting cases with a clear source-and-English-version pair, but several powerful co-causes;
4 mixed or adjacent cases;
4 counterexamples in which major crossover occurred without an English lyric.
The complete coded corpus and source ledger accompany the research edition. The public bibliography links to the principal evidence and states the limits of the method.
3.2. Scale of causal confidence
Grade
A
Meaning
A source directly documents a before-and-after sequence or an obvious new market stage following adaptation
Permitted conclusion
Adaptation was very likely one of the decisive factors
Grade
B
Meaning
The source, English version and major outcome are connected, but performer, production and distribution cannot be separated
Permitted conclusion
Adaptation is a plausible strong co-factor
Grade
C
Meaning
A hybrid or adjacent example
Permitted conclusion
The case illustrates the mechanism but does not isolate English
Grade
D
Meaning
A counterexample
Permitted conclusion
English is not a necessary condition
3.3. Limitations
The study has five principal limitations:
Survivorship bias: famous adaptations are easier to find than thousands of forgotten failures.
Selection bias: the corpus is not a random sample.
Confounding: artist, arrangement, label, radio and historical moment often change at the same time as language.
Historical incompatibility: charts from the 1950s, sales from the 1990s and streaming from the 2020s measure different markets.
No guarantee of transfer: the Russian-language RIGLI catalog proves that the team can build a language bridge. It does not prove that the English-language market will respond in the same way.
These limits do not destroy the thesis. They change its level from a guaranteed hit formula to a historically validated and experimentally testable route.