Source A: Herodotus, Histories 7.186-187
Herodotus has added up all the forces Xerxes brought with him.
'So Xerxes, son of Darius, led five million, two hundred and eighty-three thousand, two hundred and twenty men as far as Sepias and Thermopylae. That is the number of his whole force; but as for the women who baked the bread, the concubines and the eunuchs, nobody could give an accurate number ... So I am not at all surprised that some of the rivers ran dry.'
To what extent can we trust Herodotus' account of the battle of Thermopylae? Use Source A as a starting point and your own knowledge.
Model answer
Source A suggests that we should be cautious. Herodotus claims that Xerxes brought exactly 5,283,220 men to Sepias and Thermopylae, an impossibly precise and enormous figure; no ancient army could have been fed or supplied on that scale, and modern historians think it was far smaller. His comment that 'some of the rivers ran dry' shows that he accepted dramatic stories which made the Greek victory seem more miraculous. On the other hand, he admits that some numbers could not be known 'accurately', showing that he was aware of the limits of his evidence.
In other ways Herodotus deserves trust. He wrote within living memory of the battle and questioned people who remembered it; he says he learned the names of all 300 Spartans, and he quotes inscriptions that stood at the site. His description of the pass - the Phocian wall at the Middle Gate, the hot springs, the mountain path from the Asopus gorge to Alpenoi - fits the landscape, and arrowheads found on the small hill support his account of the last stand. He often gives alternative versions, for example of why the allies left, and tells us which he prefers.
However, there are weaknesses. He includes speeches, such as Demaratus' conversations with Xerxes, which he could not have heard; he includes oracles and anecdotes labelled 'it is said', such as Xerxes leaping from his throne; and his informants were Greek, often Spartan, so the Spartans appear as heroes and the Thebans as traitors, a bias that Plutarch later criticised.
Overall, we can trust Herodotus for the outline of the battle, its geography and its outcome, but not for his numbers, his speeches or his interpretation of motives, which reflect the patriotic legend that grew up after 480 BC.
Explanation
A top answer uses Source A as evidence of a weakness (exaggerated numbers) and balances it against his strengths (inquiry, inscriptions, topography, alternative versions), ending with a judgement that distinguishes outline from detail. Knowledge of Herodotus' date, methods and informants is essential for AO1. Simply calling him 'biased' without explaining how is not enough.