Measures of Central Tendency and Dispersion

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Question : 131 of 211

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Which of the following formula for standard deviation of a frequency distribution is not correct?

$s=√{\;{1}/{N} ∑ f_{i}(x_{i}-\ov{x})^{2}}$
$s=√{\;{1}/{N} ∑ f_{i} x_{i}^{2}-\ov{x}^{2}}$
$σ=√{\;{1}/{ {N}} ∑↙{ {i}} {f}_{ {i}} {x}_{ {i}}^{2}-(\;{∑ {f}_{ {i}} {x}_{ {i}}}/{ {N}})^{2}}$
$σ=√{\;{1}/{ {N}}(∑↙{ {i}} {f}_{ {i}} {x}_{ {i}}^{2})^{2}-\;{∑ {f}_{ {i}} {x}_{ {i}}}/{ {N}}}$

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