Creates Youtube-survey doc page

docs/api/covidcast-signals/fb-survey.md

@@ -126,7 +126,8 @@ our survey, this means a and (b or c).
 
 COVID-like illness or CLI is not a standard indicator. Through our discussions
 with the CDC, we chose to define it as: fever along with cough or shortness of
-breath or difficulty breathing.
+breath or difficulty breathing. From the list of symptoms from Q1 on
+our survey, this means a and (c or d or e).
 
 Symptoms alone are not sufficient to diagnose influenza or coronavirus
 infections, and so these ILI and CLI indicators are *not* expected to be

docs/api/covidcast-signals/youtube-survey.md

@@ -0,0 +1,175 @@
+---
+title: Youtube Survey
+parent: Inactive Signals
+grand_parent: COVIDcast Main Endpoint
+---
+
+[//]: # (code at https://github.com/cmu-delphi/covid-19/tree/deeb4dc1e9a30622b415361ef6b99198e77d2a94/youtube)
+
+# Youtube Survey
+{: .no_toc}
+
+* **Source name:** `youtube-survey`
+* **Earliest issue available:** May 01, 2020
+* **Number of data revisions since May 19, 2020:** 0
+* **Date of last change:** Never
+* **Available for:** state (see [geography coding docs](../covidcast_geography.md))
+* **Time type:** day (see [date format docs](../covidcast_times.md))
+* **License:** [CC BY-NC](../covidcast_licensing.md#creative-commons-attribution-noncommercial)
+
+## Overview
+
+This data source is based on a short survey about COVID-19-like illness
+run by the Delphi group at Carnegie Mellon.
+[Youtube directed](https://9to5google.com/2020/04/29/google-covid-19-cmu-research-survey/)
+a random sample of its users to these surveys, which were
+voluntary. Users age 18 or older were eligible to complete the surveys, and
+their survey responses are held by CMU. No individual survey responses are
+shared back to Youtube.
+
+This survey was a pared-down version of the
+[COVID-19 Trends and Impact Survey (CTIS)](../../symptom-survey/),
+collecting data only about COVID-19 symptoms. CTIS is much longer-running
+and more detailed, also collecting belief and behavior data. CTIS also reports
+demographic-corrected versions of some metrics. See our
+[surveys page](https://delphi.cmu.edu/covid19/ctis/) for more detail
+about how CTIS works.
+
+The two surveys report some of the same metrics. While nominally the same,
+note that values from the same dates differ between the two surveys for
+[unknown reasons](#limitations).
+
+As of late April 2020, the number of Youtube survey responses we received each
+day was 4-7 thousand. This was not enough coverage to report at finer
+geographic levels, so this indicator only reports at the state level. The
+survey ran from April 21, 2020 to June 17, 2020, collecting about 159
+thousand responses in the United States in that time.
+
+We produce [influenza-like and COVID-like illness indicators](#ili-and-cli-indicators)
+based on the survey data.
+
+## Table of Contents
+{: .no_toc .text-delta}
+
+1. TOC
+{:toc}
+
+## Survey Text and Questions
+
+The survey contains the following 5 questions:
+
+1. In the past 24 hours, have you or anyone in your household experienced any of the following:
+   - (a) Fever (100 °F or higher)
+   - (b) Sore throat
+   - (c) Cough
+   - (d) Shortness of breath
+   - (e) Difficulty breathing
+2. How many people in your household (including yourself) are sick (fever, along with at least one other symptom from the above list)?
+3. How many people are there in your household (including yourself)?
+4. What is your current ZIP code?
+5. How many additional people in your local community that you know personally are sick (fever, along with at least one other symptom from the above list)?
+
+
+## ILI and CLI Indicators
+
+We define COVID-like illness (fever, along with cough, or shortness of breath,
+or difficulty breathing) or influenza-like illness (fever, along with cough or
+sore throat) for use in forecasting and modeling. Using this survey data, we
+estimate the percentage of people (age 18 or older) who have a COVID-like
+illness, or influenza-like illness, in a given location, on a given day.
+
+| Signals | Description |
+| --- | --- |
+| `raw_cli` and `smoothed_cli` | Estimated percentage of people with COVID-like illness <br/> **Earliest date available:** 2020-04-21 |
+| `raw_ili` and `smoothed_ili` | Estimated percentage of people with influenza-like illness <br/> **Earliest date available:** 2020-04-21 |
+
+Influenza-like illness or ILI is a standard indicator, and is defined by the CDC
+as: fever along with sore throat or cough. From the list of symptoms from Q1 on
+our survey, this means a and (b or c).
+
+COVID-like illness or CLI is not a standard indicator. Through our discussions
+with the CDC, we chose to define it as: fever along with cough or shortness of
+breath or difficulty breathing. From the list of symptoms from Q1 on
+our survey, this means a and (c or d or e).
+
+Symptoms alone are not sufficient to diagnose influenza or coronavirus
+infections, and so these ILI and CLI indicators are *not* expected to be
+unbiased estimates of the true rate of influenza or coronavirus infections.
+These symptoms can be caused by many other conditions, and many true infections
+can be asymptomatic. Instead, we expect these indicators to be useful for
+comparison across the United States and across time, to determine where symptoms
+appear to be increasing.
+
+
+## Estimation
+
+### Estimating Percent ILI and CLI
+
+Estimates are calculated using the
+[same method as CTIS](./fb-survey#estimating-percent-ili-and-cli).
+However, the Youtube survey does not do weighting.
+
+### Smoothing
+
+The smoothed versions of all `youtube-survey` signals (with `smoothed` prefix) are
+calculated using seven day pooling. For example, the estimate reported for June
+7 in a specific geographical area is formed by
+collecting all surveys completed between June 1 and 7 (inclusive) and using that
+data in the estimation procedures described above. Because the smoothed signals combine
+information across multiple days, they have larger sample sizes and hence are
+available for more locations than the raw signals.
+
+## Lag and Backfill
+
+This indicator has a lag of 2 days. Reported values can be revised for one
+day (corresponding to a lag of 3 days), due to how we receive survey
+responses. However, these tend to be associated with minimal changes in
+value.
+
+
+## Limitations
+
+When interpreting the signals above, it is important to keep in mind several
+limitations of this survey data.
+
+* **Survey population.** People are eligible to participate in the survey if
+  they are age 18 or older, they are currently located in the USA, and they are
+  an active user of Youtube. The survey data does not report on children under
+  age 18, and the Youtube adult user population may differ from the United
+  States population generally in important ways. We don't adjust for any
+  demographic biases.
+* **Non-response bias.** The survey is voluntary, and people who accept the
+  invitation when it is presented to them on Youtube may be different from
+  those who do not.
+* **Social desirability.** Previous survey research has shown that people's
+  responses to surveys are often biased by what responses they believe are
+  socially desirable or acceptable. For example, if it there is widespread
+  pressure to wear masks, respondents who do *not* wear masks may feel pressured
+  to answer that they *do*. This survey is anonymous and online, meaning we
+  expect the social desirability effect to be smaller, but it may still be
+  present.
+
+Whenever possible, you should compare this data to other independent sources. We
+believe that while these biases may affect point estimates -- that is, they may
+bias estimates on a specific day up or down -- the biases should not change
+strongly over time. This means that *changes* in signals, such as increases or
+decreases, are likely to represent true changes in the underlying population,
+even if point estimates are biased.
+
+### Privacy Restrictions
+
+To protect respondent privacy, we discard any estimate that is based on fewer than 100 survey responses. For
+signals reported using a 7-day average (those beginning with `smoothed_`), this
+means a geographic area must have at least 100 responses in 7 days to be
+reported.
+
+This affects some items more than others. It affects some geographic areas
+more than others, particularly areas with smaller populations. This affect is
+less pronounced with smoothed signals, since responses are pooled across a
+longer time period.
+
+
+## Source and Licensing
+
+This indicator aggregates responses from a Delphi-run survey that is hosted on the Youtube platform.
+The data is licensed as [CC BY-NC](../covidcast_licensing.md#creative-commons-attribution-noncommercial).