Optimists and pessimists

Sir, – I note that the article "It's nice if your glass is always half full, but are you deluding yourself?" (Joe Humphreys, Unthinkable, January 20th) includes the time-worn illustration of the half-full/half-empty glass. As a scholar of operations management, can I advise that the glass is merely operating at 50 per cent capacity. – Yours, etc,

Prof MICHAEL NELSON,

Stillorgan, Co Dublin.

Sir, – Should the glasses be described as both full as they are half water and half air? – Yours, etc,

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BRID FITZPATRICK,

Dublin 6W.