"product variance" meaning in English

{
  "forms": [
    {
      "form": "product variances",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "product variance (plural product variances)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "ref": "1934, A. C. Aitken, \"Note on Selection from a Multivariate Normal Population\", Proceedings of the Edinburgh Mathematical Society 4, page 106",
          "text": "From a population characterised by variables x,y,z, correlated and normally distributed, with means 0,0,0, variances σ₁²,σ₂²,σ₃² and product variances r_12σ₁σ₂,r_13σ₁σ₃,r_23σ₂σ₃, a sub-population is extracted […] ."
        }
      ],
      "glosses": [
        "Covariance."
      ],
      "id": "en-product_variance-en-noun-tS4FcarD",
      "links": [
        [
          "Covariance",
          "covariance#English"
        ]
      ],
      "raw_glosses": [
        "(obsolete or rare) Covariance."
      ],
      "tags": [
        "obsolete",
        "rare"
      ]
    }
  ],
  "word": "product variance"
}

{
  "forms": [
    {
      "form": "product variances",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "product variance (plural product variances)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with obsolete senses",
        "English terms with rare senses"
      ],
      "examples": [
        {
          "ref": "1934, A. C. Aitken, \"Note on Selection from a Multivariate Normal Population\", Proceedings of the Edinburgh Mathematical Society 4, page 106",
          "text": "From a population characterised by variables x,y,z, correlated and normally distributed, with means 0,0,0, variances σ₁²,σ₂²,σ₃² and product variances r_12σ₁σ₂,r_13σ₁σ₃,r_23σ₂σ₃, a sub-population is extracted […] ."
        }
      ],
      "glosses": [
        "Covariance."
      ],
      "links": [
        [
          "Covariance",
          "covariance#English"
        ]
      ],
      "raw_glosses": [
        "(obsolete or rare) Covariance."
      ],
      "tags": [
        "obsolete",
        "rare"
      ]
    }
  ],
  "word": "product variance"
}