In Roger Broad's Conscription in Britain, 1939-1964: The Militarisation of a Generation (Taylor & Francis, 2006) is found this sentence:
In 1914–18, 30 per cent of all Britain's soliders were wounded at least once; in 1939–45, 6 per cent. The chances of recovery were much better. … Psychiatric treatments had also advanced. The progress from the dismissive 'cowardice/malingering' to 'shellshock' in the first war, through 'battle fatigue to 'post-traumatic stress disorder' parallels the recognition of the complexity of the condition.
The comparative incidence of psychiatric injury in between the two conflicts cannot be known, for both [the] definition and [the] treatment changed radically over those years."
Even in the Second World War the recognition of battle stress was long resisted by some military authorities: the belief that the problem was 'malingering' rather than combat-induced psychiatric disability had a long life.
Why did the author omit the definite article in this sentence? I added them there myself. To me, it sounds awkward and disjointed without the definite article.