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<title>Estimate a limit of quantification (LOQ) — loq • chemCal</title>
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<h1>Estimate a limit of quantification (LOQ)</h1>
</div>
<p>The limit of quantification is the x value, where the relative error
of the quantification given the calibration model reaches a prespecified
value 1/k. Thus, it is the solution of the equation
$$L = k c(L)$$
where c(L) is half of the length of the confidence interval at the limit L
(DIN 32645, equivalent to ISO 11843). c(L) is internally estimated by
<code><a href='inverse.predict.html'>inverse.predict</a></code>, and L is obtained by iteration.</p>
<pre>loq(object, …, alpha = 0.05, k = 3, n = 1, w.loq = "auto",
var.loq = "auto", tol = "default")</pre>
<h2 class="hasAnchor" id="arguments"><a class="anchor" href="#arguments"></a> Arguments</h2>
<dl class="dl-horizontal">
<dt>object</dt>
<dd>
A univariate model object of class <code>lm</code> or
<code><a href='http://www.rdocumentation.org/packages/MASS/topics/rlm'>rlm</a></code>
with model formula <code>y ~ x</code> or <code>y ~ x - 1</code>,
optionally from a weighted regression. If weights are specified
in the model, either <code>w.loq</code> or <code>var.loq</code> have to
be specified.
</dd>
<dt>alpha</dt>
<dd>
The error tolerance for the prediction of x values in the calculation.
</dd>
<dt>…</dt>
<dd>
Placeholder for further arguments that might be needed by
future implementations.
</dd>
<dt>k</dt>
<dd>
The inverse of the maximum relative error tolerated at the
desired LOQ.
</dd>
<dt>n</dt>
<dd>
The number of replicate measurements for which the LOQ should be
specified.
</dd>
<dt>w.loq</dt>
<dd>
The weight that should be attributed to the LOQ. Defaults
to one for unweighted regression, and to the mean of the weights
for weighted regression. See <code><a href='massart97ex3.html'>massart97ex3</a></code> for
an example how to take advantage of knowledge about the
variance function.
</dd>
<dt>var.loq</dt>
<dd>
The approximate variance at the LOQ. The default value is
calculated from the model.
</dd>
<dt>tol</dt>
<dd>
The default tolerance for the LOQ on the x scale is the value of the
smallest non-zero standard divided by 1000. Can be set to a
numeric value to override this.
</dd>
</dl>
<h2 class="hasAnchor" id="value"><a class="anchor" href="#value"></a>Value</h2>
<p>The estimated limit of quantification for a model used for calibration.</p>
<h2 class="hasAnchor" id="note"><a class="anchor" href="#note"></a>Note</h2>
<p>- IUPAC recommends to base the LOQ on the standard deviation of the signal
where x = 0.
- The calculation of a LOQ based on weighted regression is non-standard
and therefore not tested. Feedback is welcome.</p>
<h2 class="hasAnchor" id="see-also"><a class="anchor" href="#see-also"></a>See also</h2>
<p>Examples for <code><a href='din32645.html'>din32645</a></code></p>
<h2 class="hasAnchor" id="examples"><a class="anchor" href="#examples"></a>Examples</h2>
<pre class="examples"><div class='input'><span class='fu'>data</span>(<span class='no'>massart97ex3</span>)
<span class='fu'>attach</span>(<span class='no'>massart97ex3</span>)
<span class='no'>m</span> <span class='kw'><-</span> <span class='fu'>lm</span>(<span class='no'>y</span> ~ <span class='no'>x</span>)
<span class='fu'>loq</span>(<span class='no'>m</span>)</div><div class='output co'>#> $x
#> [1] 13.97764
#>
#> $y
#> 1
#> 30.6235
#> </div><div class='input'>
<span class='co'># We can get better by using replicate measurements</span>
<span class='fu'>loq</span>(<span class='no'>m</span>, <span class='kw'>n</span> <span class='kw'>=</span> <span class='fl'>3</span>)</div><div class='output co'>#> $x
#> [1] 9.971963
#>
#> $y
#> 1
#> 22.68539
#> </div></pre>
</div>
<div class="col-md-3 hidden-xs hidden-sm" id="sidebar">
<h2>Contents</h2>
<ul class="nav nav-pills nav-stacked">
<li><a href="#arguments">Arguments</a></li>
<li><a href="#value">Value</a></li>
<li><a href="#note">Note</a></li>
<li><a href="#see-also">See also</a></li>
<li><a href="#examples">Examples</a></li>
</ul>
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