The essential skills used to obtain best advanced search results in CINAHL are $...$

a$.$

PICO keywords, synonyms, truncation, combiners, specificity sequence, refinement,

b$.$

PICO keywords, synonyms, subject headings, Boolean AND$/$OR$,$ truncation, refinement.

c$.$

PICO keywords, smart$-$text, subject headings, Boolean AND$/$OR$,$ truncation, refinement.

d$.$

PICO keywords, synonyms, truncation, combiners, sensitivity sequence, refinement.