Data from: Measurement artefacts lead to false positives in the study of birdsong in noise

1: Numerous studies over the past decade have reported correlations between elevated levels of anthropogenic noise and a rise in the minimum frequency of acoustic signals of animals living in noisy habitats. This pattern appears to be occurring globally, and higher pitched signals have been hypothesized to be adaptive changes that reduce masking by low-frequency traffic noise. However, the sound analysis methods most often used in these studies are prone to measurement errors that can result in false positives. In addition, the commonly used method of measuring frequencies visually from spectrograms might also lead to observer-expectancy biases that could exacerbate measurement errors.
2: We conducted an experiment to (a) quantify the size and type of errors that result from “eye-balling” frequency measurements with cursors placed manually on spectrograms of signals recorded in noise and no-noise conditions, and (b) to test whether observer expectations lead to significant errors in frequency measurements. We asked 54 volunteers, blind to the true intention of our study, to visually measure the minimum frequency of a variety of natural and synthesized bird sounds, recorded either in noise, or no-noise conditions. Test subjects were either informed or uninformed about the hypothesized results of the measurements.
3: Our results demonstrate that inappropriate methodology in acoustic analysis can yield false positives with effect sizes as large, or even larger, than those reported in published studies. In addition to these measurement artefacts, psychological observer biases also led to false positives – when observers expected signals to have higher minimum frequencies in noise, they measured significantly higher minimum frequencies than uninformed observers, who had not been primed with any expectation.
4: The use of improper analysis methods in bioacoustics can lead to the publication of spurious results. We discuss alternative methods that yield unbiased frequency measures and we caution that it is imperative for researchers to familiarize themselves both with the functions and limitations of their sound analysis programs.. In addition, observer expectancy biases are a potential source of error not only in the field of bioacoustics, but in any situation where measurements can be influenced by human subjectivity.

1: 过去十年间，大量研究均报道了人为噪声（anthropogenic noise）水平升高与嘈杂生境中动物声学信号最低频率上升之间的相关性。这一模式似乎在全球范围内普遍存在，学界曾假设更高频的信号是一种适应性变化，可降低低频交通噪声的掩蔽（masking）效应。然而，此类研究中最常用的声学分析方法往往存在测量误差，可能导致假阳性（false positives）结果。此外，从声谱图（spectrogram）上手动放置光标目视读取频率的常用方法，还可能引入观察者期望偏差（observer-expectancy biases），进一步加剧测量误差。
2: 我们开展了一项实验，旨在(a)量化通过手动将光标置于声谱图（spectrogram）上"目测"频率测量值所产生的误差类型与大小，实验涵盖噪声与无噪声条件下录制的信号；(b)检验观察者期望是否会导致频率测量出现显著误差。我们邀请了54名志愿者，且未告知他们本研究的真实意图，要求他们目视测量多种自然及合成鸟鸣声的最低频率，这些鸟鸣声均录制于噪声或无噪声环境中。测试受试者被分为两组：一组被告知该测量实验的假设性结果，另一组则未获得相关信息。
3: 本研究结果表明，声学分析中使用的不当方法可产生假阳性（false positives）结果，其效应量（effect sizes）甚至与已发表研究中报道的相当，乃至更大。除上述测量伪迹（artefacts）之外，心理层面的观察者期望偏差同样会引发假阳性结果：当观察者预期噪声环境中的信号具有更高的最低频率时，他们测得的最低频率会显著高于未被预设任何预期的不知情受试者。
4: 生物声学领域若使用不当的分析方法，可能导致发表虚假结果（spurious results）。我们讨论了可生成无偏频率测量值的替代分析方法，并提醒研究者必须熟悉其声学分析软件的功能与局限性。此外，观察者期望偏差不仅是生物声学领域的潜在误差来源，在任何可能受人类主观性影响的测量场景中，均存在此类误差风险。