January 30, 2019

Evaluating Descriptive Claims

Plan for Today:

(1) Recap: Levels of Measurement

(2) Validity

Recap

Levels of Measurement

The kinds of values taken by a variable is called its level of measurement

We consider four levels of measurement

  • Nominal
  • Ordinal
  • Interval
  • Ratio

Not to be confused with measures

Levels of measurement

nominal: values place cases into unranked categories

  • religion, party affiliation, regime type

ordinal: values place cases into categories that are ranked in some way, without meaningful intervals

  • University rankings, Ideology (very left, somewhat left, neither, somewhat right, very right)

interval:: values both rank the cases and have consistent intervals, but zero and ratio not meaningful

  • Year/Date, Temperature (in Celsius, but not Kelvin), ideological scores (-1 to 1) of legislators

ratio: values rank the cases, have consistent intervals, and give a meaning to values of 0 and ratios

  • Time since some event, Counts of events, Rates (e.g. unemployment)

Levels of measurement: Practice

Levels of measurement: Practice

If we are measuring "democracy", what is the level of measurement of…

1. The Year in which a country became a democracy

2. Number of Years since a country became a democracy

3. Fraction of adults who can are allowed to vote under suffrage rules

Levels of measurement: Practice

If we are measuring "democracy", what is the level of measurement of…

1. The Year in which a country became a democracy (interval)

2. Number of Years since a country became a democracy (ratio)

3. Fraction of adults who can are allowed to vote under suffrage rules (ratio)

Levels of measurement: Practice

Choosing the right variable:

absolute values are counts given in raw units

  • Examples:
    • dollar amounts
    • Number of events
    • Number of deaths

relative values are given in fractions or rates or ranks

  • Examples:
    • Units are fractional (deaths/population, events/time)
    • No units (percentile, rankings)

Choosing the right variable:

Suppose we want to compare the US and Canada to find out which has more widespread gun violence

Which variable would be best?

  1. Number of gun deaths in the preceding year
  2. Rank of the country in a list of gun deaths in the preceding year
  3. Gun deaths per 100,000 people.

Validity

Variables and Measures can fail:

Even if we develop a useful concept

Variables or measures may not correspond to the concept

Racial Bias in Police Shootings

Is police violence in the US racially biased against African Americans?

Need a concept for "racial bias":


"Disparate treatment of a racial group in excess of statistical discrimination"

  • if unequal treatment of African Americans were based solely on statistical discrimination…

  • then African Americans treated identically to white Americans in otherwise identical situations.

Racial Bias in Police Shootings

Concept: "racial bias in police shooting"

to see if bias is in excess of statistical discrimination…

  • Dimension: "objective threat" to the officer

Variable:

"the level of physically-threatening behavior exhibited by a suspect"

Measure:

undergraduate research assistants code threatening behavior by suspects given in police reports

Measurement Trouble: Validity

validity:

  • Degree of fit between a variables or its measure and the concept the variable is intended to capture.
  • When a variable and its measure "capture" or "map onto" the concept we are interested in, then we say they have "validity"
  • When a variable and its measure "capture" or "map onto" other concepts we are not interested in, then we say they lack "validity"

Measurement Trouble: Validity

Threats to validity

Validity can break down in two places:

  1. Concept \(\xleftarrow{Mismatch}\) Variable
  2. Variable \(\xleftarrow{Mismatch}\) Measure

One or both of these could happen.

Measurement Trouble: Validity

Threats to validity

In the racial bias example…

The measure might not reflect the true level of objective physical threat…

  • Police may misperceive higher threats from African Americans; Police may actively exaggerate threats from African Americans in police reports.
  • measure might map onto concept "objective threat" but it might also map onto "racial stereotypes".


Variable \(\xleftarrow{Mismatch}\) Measure

Measurement Trouble: Validity

"Which country/province is most politically corrupt?"

Concept: Political Corruption or "the use of power by government officials for illegitimate private gain"

Variable: (for a place) Fraction of politicians in a place prosecuted for corruption

Measure: Match criminal court proceedings to list of politicians for a given place.

Does this capture anything inside the concept?

  • Places with lots of corruption do not prosecute corruption
  • Places with low corruption successfully prosecute corruption
  • But the measure gives correct values for the variable

Concept \(\xleftarrow{Mismatch}\) Variable

Measurement Trouble: Validity

"How often are guns used for self-defense?"

Concept: Self Defense Gun Use or "resistance to a crime through display or use of a firearm"

Variable: Number of events where people believe themselves to be victims of a crime and use a firearm to defend themselves.

Measure: Survey people about their use of firearms, ask if they used them in self-defense.

Problems

  • people misperceive crimes that are not happening (different from mis-reporting)
  • Survey respondents misreport threat or use of a weapon (different from mis-perception)

Measurement Trouble: Validity

"How often are guns used for self-defense?"

Concept: Self Defense Gun Use or "resistance to a crime through display or use of a firearm"

Variable: Number of events where people believe themselves to be victims of a crime and use a firearm to defend themselves.

Measure: Survey people about their use of firearms, ask if they used them in self-defense.

Problems

Concept \(\xleftarrow{Mismatch}\) Variable

Variable \(\xleftarrow{Mismatch}\) Measure

Threats to validity

  1. Measure/Variable does not cover enough of the concept:
    • Measure only captures some but not all relevant dimensions of the concept
  2. Measure/Variable covers things outside the concept:
    • Could cover somethings inside the concept, or nothing inside the concept
    • e.g. fraction of politicians convicted of corruption
    • e.g. survey of reported self-defense gun uses
  3. Measure captures different things across units: non-comparability
  • e.g. police assessment of "objective threat" across races

Threats to validity

Concept: Exposure to political information

Variable: Frequency of reading a newspaper

Measure: Survey of people asking for frequency with which they read a newspaper

Does this capture enough of the concept?

  • If we want to compare exposure to political information over time, this could be a problem
  • In the 1940s/1950s, newspapers major source of information, but now televisiona nd internet more common.
  • Breakdown at Concept/Variable

Summary

Validity

Pertains to the link between our observations and the concept we want those observations to capture.

  • Can fail because we have chosen a variable that insufficiently captures the concept, maps onto other (unhelpful) concepts, or captures different things for different concepts.

  • Can fail because our measure does not yield the correct values for our variable and instead reflect other concepts