Having just briefly looked at both in the OED there seems to me plenty of room for interchangeability in the way they are used.
Under 'retrospective' for example there is an instance of legislation having been passed which took effect from an earlier time. This would seem to me as though it should be 'retroactive'.
And interestingly both words carry meanings which include the other. Can anyone summarise the difference by way of simple examples as to their separate uses?
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retrospective, adj. and n.
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Pronunciation: Brit. /ˌrɛtrə(ʊ)ˈspɛktɪv/ , U.S. /ˌrɛtrəˈspɛktɪv/
Etymology: < post-classical Latin retrospect-, past participial stem of retrospicere to look back (see retrospect v.) + -ive suffix. With the use as adjective compare French rétrospectif (1779), Spanish retrospectivo (1587). With the use as noun compare earlier retrospect n., retrospection n., and (with the specific use in sense B. 2) French rétrospective (1920 in this sense; 1919 in sense ‘screening of a series of old films’), Spanish retrospectiva (1872 in this sense). Compare earlier retrospectively adv., which may imply earlier currency of the adjective.
N.E.D. (1908) also gives the pronunciation (rītrospe·ktiv) /riːtrəʊˈspɛktɪv/ .
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A. adj.
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- Chiefly Law. Taking effect from a date in the past; retroactive.
1660 F. Philipps Tenenda non Tollenda iii. 110 The new mode of making retrospective Acts of Parliament.
a1768 J. Erskine Inst. Law Scotl. (1773) I. ii. vii. §15 It hinders the confirmation from having that retrospective quality.
1788 Considerations submitted to House of Lords on Two East-India Bills 54 All Acts of the Legislature, are retrospective, as well as prospective.
1843 Macaulay Crit. & Hist. Ess. I. 465 Sentencing a man to death by retrospective law.
1873 Sat. Rev. 9 Aug. 36/1 It is a retrospective alteration of a contract by one of the parties to the detriment of the other.
1899 T. C. Allbutt et al. Syst. Med. VIII. 321 The amnesia, however, is not, so to speak, retrospective.
1934 Yale Law Jrnl. 44 360 The arguments on the legality of retrospective legislation have been impressive in variety, but unconvincing in substance.
1955 Times 28 June 6/5 The decision was retrospective to April 1, when the new rates came into operation.
2004 H. Kennedy Just Law (2005) iii. 82 The retrospective nature of the intended legislation makes it doubly wrong.
Above is meaning number one (or retrospective) from the OED. Clearly it encompasses the 'retroactive'. And some of the more recent examples are clearly ones where you would claim 'retroactive' were appropriate. It seems to me that in Briatain, at least, 'retrospective' has to some extent taken on the meaning of 'retroactive'.
Best Answer
While they sometimes may be used interchangeably, retrospective is primarily a looking quality while retroactive is primarily doing.
You can have a retrospective show of an artist's work (looking back over her career). You would not have a retroactive show.
Similarly you can have a retrospective analysis (reviewing past activities), but not a retroactive analysis.
When an action is intended to be effective based on an earlier period, the term retroactive is more often used (at least in the US). In effect, you are saying even though it happens now, we are treating it as if it happened then.
However, you may say we are taking action now and acting based on a retrospective computation. This is arguably different from a retroactive event. The former is treated as a present activity and the latter is treated as an activity that relates back to an earlier time (nunc pro tunc for the Latin/legal geeks).