The noun of 'sparse' — the fact or the quality of being thin on the ground. You use it when you want to talk about the thinness itself as a thing: 'the sparseness of the data', 'the sparseness of the furniture', 'the sparseness of the population'. It almost always appears in the shape 'the sparseness of X'. This is a formal, written word — reports, research papers, serious journalism. In speech people simply say 'there wasn't much data'. One thing worth knowing: writers sometimes use it as a compliment. 'The sparseness of his prose' means his writing is bare and stripped down, with nothing extra, and the writer usually admires that. So sparseness can mean a lack you regret, or a bareness someone chose on purpose. The sentence around it tells you which. You may also meet 'sparsity', which is the same idea and is the form preferred in mathematics and computing — a sparse matrix, sparsity of data. Outside those fields, 'sparseness' is the more ordinary of the two.

稀疏、稀少（抽象名詞；數學和電腦領域多用 sparsity）